Proven Guilty final iteration.

[morriswalters]

How did the Gatekeeper know that Molly had been using Black Magic?
Because he's been watching Chicago for Mab.  How do we know he can do something like that.  Jim tells you by what he does at the end of the trial, just before the team blows in with the newbee's.  He watches the running fight involving Eb and the other Council members.

Why is she watching?
Because Lea had been watching and protecting Harry, and Mab had confined Lea.

How is she watching or the Gatekeeper watching?
He or she are using the magic of the Outer Gates.

How did she know of Little Chicago and the fact that it was broken?
The Mothers told her.  As when Harry learned the name of the Adversary, the act of Harry dying in the first use spawns darker futures.  This may work for Mab.

Why was Pell attacked?
To clear the way for Harry to come to Arctis Tor. And to Bring Harry in.

Why have Harry come to Arctis Tor?
To see Lea and Slate and the after effects of the Black Council attack and to sus out a traitor among Winter. It explains why she didn't declare war on the Red Court after the events of Dead Beat. She has decided that Harry must be the next Winter Knight and recognizes the attack on Molly as a ploy to put Harry at war with the White Council, which would result in his death.  Bringing Molly to Arctis Tor gives her an opportunity to control the encounter.

Why did the Black Council attack?
To force Winter to defend the border. Since with Lea and Slade on ice, Mab is weak, she must remain in the fortress to protect the Wellspring, while her forces guard the border.  This allows the Red's to attack within faerie with impunity during and after the events of Dead Beat.  Mother Summer tells you why she has her troops at the border.  Jim obfuscates the time of attack since Dead Beat was originally to be published after Proven Guilty.

Who attacked Pell?
Mab causes Pell to be attacked, which causes Molly's boyfriend Nelson to get locked up. We can infer this because they target Pell and no one else, and no one is killed. Had Pell died Nelson couldn't have gotten bail and Harry wouldn't have been needed. Since Molly is estranged from her parents, she has no one else to call to get help for Nelson but Harry.  This is all a distraction to keep Harry from using Little Chicago too soon.  It also points him towards Nelson and Molly.

Who did the second attack?
Maybe Maeve.  She uses similar magic against Slate in Summer Knight.  It may have been an attempt to kill Sandra Marling as a cutout.  Marling  may have been the person running from the room.  She also gets Molly away from the con before the third attack. And it may be that she sent the Scarecrow after Glau for a similar reason.

Who did the third attack? 
Maeve.  This was the money attack which had been planned from the start.  This was to call the attention of the White Council to Molly.  It is important to know here that the lights are killed mechanically.  It's dark but there is no myrk.  Maeve had to send them through, since Molly as the beacon, was no longer there.  Harry's spell sent them to Molly.  I speculate that Sandra Marling rigged the lights before she split.

Who is Sandra Marling?
Goodman Grey.  Why.  Because I like him.

[Bad Alias]

I don't see why the Outer Gates would be some remote viewing device. I don't know why this is even an idea, let alone a somewhat popular one around here.

We do know the Gatekeeper is super mysterious, even to others on the Supreme Council, and that he can do some remote observing and future predicting. We've seen him predict the future in Turn Coat, and it was a different future than he had predicted before Harry either conducted the sanctum ritual or stepped on the island. (We don't know when he made his first prediction that if Harry stood up to the Council he would die). In Summer Knight, he told Harry that he hadn't been watching him so much as, if I recall correctly, listening to him.

[morriswalters]

In Cold Days when discussing the Gates with Rashid, Harry says its a CAT scanner for Outsiders, and Rashid responds, 'Among many other things'.  It appears to be the device in the book around which everything is built.

Rashid is the international man of mystery.  One who can do the impossible and can locate Black Magic.  Not to mention Harry, at will.  Something Mab is able to do as well. 

I also think it is the means used in Cold Days to shut down the border of faerie to portals.  In Proven Guilty during Molly's trial  Rashid appears to keep track of the running fight between Eb and the others as they progress towards Chicago and his eye is linked to the Gate.  And Mab watches as Marcone is kidnapped in Small Favor, and Gard can sense her watching.

And my new WAG reflects the alternate explanation given to Harry about the events by Michael.

[Yuillegan]

An interesting WAG.

I am not sure whether the Gates are a "remote viewing device" per se...I suspect their real use is to do with the Greater Multiverse. My theory is, like Lovecraft's silver gate to the multiverse, the crystal material of the Gates, the Gatekeepers eye etc. is connected to the very fabric of reality. This allows certain beings to see into the various universes, and therefore alternate and parallel timelines. This allows the Gatekeeper (and perhaps Mab and particularly the Mothers) to "see" what is going on, before it is going to happen. It is not logical that all universes are in sync and are at the same point in history. Time as we know (especially in the Dresden Files) is linear to a human perspective. So realistically this knowledge allows them to interpret the most likely version of an event about to happen, in order to take actions. And I believe Vadderung has still some of this ability (although not as much as he probably used to...I think there is a reasonable argument that it is implied that he once wore the Gatekeeper eye).

Basically, less "see" other points in space in Dresden's universe and more "see" other universes that are/have happening(ed) and infer what might happen in "their" universe. Complicated I know, but spacetime and multiverses are by their very nature stupidly complex.

Now onto your other points.

Perhaps the Gates are the mechanism to close the borders - it fits as well as anything. They are on some of the borders of Faerie (but of course not on the border that is shared with Summer. But one wonders if Faerie isn't so much split into North and South for example (with let's say Winter in the North and Summer in the South) but actually the territory of Winter surrounding the centre of Faerie (which is Summer's territory). And presumably in the No Mans Land inbetween the territories is the realms of the Wyld Fae. This would make more sense if all the Borders of Faerie touch the Outside and therefore have walls etc. But then perhaps, maybe not as well. It would be nice to have a vague map. We have to assume where Dresden visited was in Winter's territory, which of course still makes it interesting that Mother Summer was there but then again those two don't really play by the regular Rules. Of course, if the Outer Wall does not surround ALL of Faerie and therefore Summer has a border with some other part of the Never-never or the mortal world, then how did Mab close all the Borders using the mechanism of the Gate? Food for thought.

1. Rashid may or may not have been remote viewing. Just as likely, if not more so, he was seeing several possible futures become one clear future and acted upon that information.
2. The why makes sense though, so fair enough.
3. See main argument above.
4. THIS. I find it very interesting that Mab kept Harry alive. Mab is Mother Winter's "daughter". She would think along similar lines - consider that scene in Cold Days about "too many bright ones [futures]". I think Harry's involvement with Mab serves to create much darker timelines, provided Harry doesn't mess up those plans We also must consider that old WOJ about Titania being nervous about Harry, and Mab keeping her enemies closer. Harry's role may not be her traditional friend. All that Starborn stuff.
5. Good enough for me, although don't be so quick to discount that this book was just as much about bringing MOLLY into Arctis Tor. She was being set up as one of their agents for a long time. Maybe for their benefit, maybe for Michael's punishment for crossing Winter, maybe other reasons. Probably all of them knowing Mab.
6. Yeah pretty sound. But see above.
7. Very interesting idea. The Time obfuscation makes sense here. However, I am not so sure that this was their only motive. Whilst I truly doubt they thought they could kill Mab, perhaps they thought they could wound her. More likely though, I think they were gunning for Leah and maybe even the knife. Why? Perhaps there was some great future value and they did not wish to lose an asset so recently gained. This was one of the boldest attacks, right at the heart of Winter's power. Although as the attack in Cold Days showed, they like to hit multiple fronts to create chaos and stretch resources. So quite possibly, this was a distraction to some other terrifying attack. Or, as the books seem to imply (if not state outright), Winter's forces kept near the borders (both the Shared Border and the Outer Gates) meant there was only an honour guard to protect Mab and her assets at Arctis Tor. And yes, this also gave the Reds mostly uninhibited access through Faerie.
8. I think this was explained as a fetch, and yes quite probably on Mab's orders. And I agree Pell survived otherwise the book doesn't work. Intersting, perhaps it does help distract from Little Chicago.
9. Maybe...but I still think it brought the hallmarks of Fetches. Which is not to say Maeve didn't help, or lead.
10. Yeah I agree pretty much with all of that.
11. Yeah sure why not. Good as any other reason.

[Kindler]

I think it's possible that Rashid wasn't detecting Black Magic at all, but merely saw some of the results of black magic use in Chicago. As in, he saw the possibility that Molly would be tried and executed by the Council. I think it's an important distinction, because I don't know of any way GateBro could detect Black Magic over an area as large as Chicago. I mean, he has to manually check the Sidhe at the Outer Gates for Nemesis. We don't know that he can really sense Black Magic taint without being at or near the location in question—but we do know that he can take peeks into the future.

[morriswalters]

Given Mab's interchange with Harry at the end of Cold Days it isn't hard to speculate that they saw what was happening because they were watching.  She knew every detail of Harry's and Molly's interaction from roughly Death Masks forward.

[g33k]

I like a LOT of this, but differ in a few key elements...

How did the Gatekeeper know that Molly had been using Black Magic?
Because he's been watching Chicago for Mab.

I do see "watching Chicago..." but I see no need for a "... for Mab."

Rashid has all sorts of oracular/scrying abilities that even the rest of the Senior Council doesn't understand.  It's not even clear that "the Gatekeeper" is a White Council role, the way "the Merlin" is... or that being Gatekeeper comes from those abilities... or that those abilities make him eligible for the role.  Uncertainty wrapped in ambiguity, packed within ignorance!

I'm pretty sure that Rashid understands was a Starborn is, though; and that Harry is one.

A Starborn wizard sitting on a nexus site like Chicago is going to get a LOT of Rashid's attention, Mab or not.  Add in the improbable amount of Bad Shit coming/sent to Chicago (from which Harry deduces the "Black Council") and I expect Rashid probably has a laser-focus on the place, the realization that the suspected issues are very real indeed.

Given his known oracular/scrying abilities, I see no reason to posit a Mab tip-off:  he's sufficiently on the ball to notice the taint of black magic, since he's already paying close attention.

Also, tipping off Harry doesn't really advance Mab's cause:  Nelson is getting arrested, Molly is about to call Harry, and the dominoes are beginning to fall.  Harry will suspect black magic immediately, even without a tip from Rashid.

So I think Rashid's warning was pretty on-the-level, and not tied into Mab's plot.

... How do we know he can do something like that.  Jim tells you by what he does at the end of the trial, just before the team blows in with the newbee's.  He watches the running fight involving Eb and the other Council members ...

I think Rashid is using his own foretelling.  He's not scrying a fight in realtime, he's watching potential-futures diverge and coalesce, new what-might-be's created, old ones wither away.  Finally he sees that a Knight of the Cross is indeed coming to the warehouse where his daughter is on trial for her life, and opens the door for him.

Why is she watching?
Because Lea had been watching and protecting Harry, and Mab had confined Lea.

Yes, absolutely!

But this is only the first half.  I will WAG that Lea was Harry's godmother because Mab wanted her to be.  Oh, sure, there was the whole bargain-with-MargaretLeFay schtick; but that was just the formula they had to go through (being faeries), and a bit of theater for the poor apeshumans to focus upon.  Mind you, Mab was indeed Lea's stand-in (at this point) because Mab was "obligated" to take up Lea's duties... but Mab wanted the role anyhow.

Because Mab already wanted Harry -- she wanted Harry to be the next WinterKnight.  Because Winter is fighting the Outsiders, and a Starborn Wizard would be a marvelous weapon to wield against them.  I mean... Lloyd Slate vs. Outsiders?  Seriously?  So she is finally beginning the process of harvesting the Starborn Wizard Winter Knight whom she has been growing (by way of Lea) for many years...

How is she watching or the Gatekeeper watching?
He or she are using the magic of the Outer Gates.

Nah.  This is our biggest difference, I think.

The Outer Gates are GATES, first and foremost.  They aren't Mab's Own Starfleet Sensor Array.  They can spot Outsiders, because that's part of their Gate-y schtick:  separating the Outside from the Inside.  Other scrying... notsomuch, no.

Unless, of course, Molly's Black Magic was in fact Outsider Magic.  Because I totally buy the Outer Gates as spotting that!   There's the whole "all Black Magic is Outsider Magic" theory, which I think would mean the Outer Gates can track it; I don't buy that theory, but I don't reject it either... I just find "insufficient evidence" either way.

But Mollie's specific mind-magic may have an Outsider taint, whether or not other forms of Black Magic do.  That whole thing where they give Faeries the ability to lie, to act with "Free Will" against their natures/mantles, hmmm?  And what's Molly doing?  Oh!  Mucking around with her victims' Free Will!  Which does seem kinda Outsider-y, hmmm?

How did she know of Little Chicago and the fact that it was broken?
The Mothers told her.

Straight up, man:  any time someone alleges "the Mothers knew that," I'm probably "yup, they did; they soooo knew that!"  So I don't disagree with you, exactly...

But I don't think that's what happened!  Here's why:

First off, "The Mothers told "  just seems... not very them, y'know?  The Mothers aren't much for "telling," I think.  And they seem especially loath to tell one of the other Queens (they evidently knew Aurora's plan, and could have told Mab and/or Titania what Aurora was planning; but evidently "couldn't" tell either Queen).

The other side from Harry's Lab is Lea's Garden; it's probably where she does experiments of her own (such as "amusing new ways to kill" and "creative desceptions"), so as to keep a close correspondence.

But then Lea had to be sidhe-sicle'd, and Mab had to take over.

You remember how impressed Harry was when Lily opened the Way in Pell's theater?  You realize how much more subtle Mab is than Lily?  How much more powerful the Winter Queen is than the Summer Lady?  How much easier the opening is (compared to the Summerlady opening to Arctis Tor) when the 2nd-strongest Winterfae has been building a correspondence for YEARS?

Do you really think ANYTHING that's in that lab at that time has escaped Mab's notice?  Mab can see into Harry's lab as easily as you or I change tabs in our browser!

Why was Pell attacked?
Why have Harry come to Arctis Tor?
Why did the Black Council attack?
Who attacked Pell?
Who did the second attack?
Who did the third attack? 

Preach it, brother!
I am marching beside you, shoulder to shoulder!  I mean, you've got some details I hadn't considered, and you left off a few I think are relevant.  But every phrase of your query/response has me going, "yep... yep... yep..."

Who is Sandra Marling?
Goodman Grey.  Why.  Because I like him.

.... aaaand, you've lost me again.   

Honestly, I see no reason to suspect Sandra Marling is anything other than a Muggle horror-movie fan (is there some WoJ or something, stating that she's more than what she seems to be?  because I see no other reason for so much suspicion around her)   But it's OK, this is a minor point!   

[morriswalters]

Honestly, I see no reason to suspect Sandra Marling is anything other than a Muggle horror-movie fan (is there some WoJ or something, stating that she's more than what she seems to be?

It isn't completely unsupported.  Somewhere there is a WOJ that says Harry and Molly looked for her after the fact.  But remove here from the story and the story falls apart.  She pulls the actors together, she sets the stage, she puts the idea of using fear as a cure, and so on and so on.  And then poof, gone like a fart in the wind.  We last see her taking Molly home.

Depending on when Jenny Greenteeth died it could have been her, I can't keep track.  I just like Goodman Grey.  But Jim spent a lot of time on her, she's important. And give some thought to this from Changes.

“La,” she said, smiling. “My guardians were created to attack any intruder—including one that looked like you. We couldn’t have some clever shapeshifter slipping by, now, could we?”

Given his known oracular/scrying abilities, I see no reason to posit a Mab tip-off:  he's sufficiently on the ball to notice the taint of black magic, since he's already paying close attention.

Who had been watching Harry all the time?  Lea, going so far as to put a monster trap on the portion of the Never Never adjacent to Harry's home?  And in Proven Guilty Mab was carrying her water.  And to top it off she had already decided to make him her Knight, thus the reason for leaving Slate hanging. From Changes

“I should appreciate such a thing.” She studied me quietly. “It has been true for your entire lifetime, child. I have followed you in the spirit world. Created guardians and defenses ’pon the other side to ward your sleep, to stand sentinel over your home. And you still have only the beginnings of an idea of how many have tried.” She smiled, showing her delicately pointed canine teeth again. “Tried, and failed.”
Which also explained how she was always near at hand whenever I had entered the Nevernever. How she would be upon my trail in seconds whenever I went in.
Because she had been there, protecting me.
From everything but herself.

So if you consider that Harry was the real target you see how they knew. Put saving Molly as the focus of the book and the pieces don't fit, put Harry in the focus and the pieces more or less fall into place.  And Michael suggests it at the end.

[KurtinStGeorge]

If Maeve was involved in any of this, it's something that pretty much can only be confirmed in a time travel Dresden story.  Yes, in theory there could come a time when Harry works out most of the details and in a conversation or dispute with Mab, she reveals her daughter was responsible for the attacks, but that wouldn't be very good writing.  It would be far better for Harry to go back in time and learn of Maeve's role that way.  Plus, many of us assume it was future Harry who who hit the blue beetle from behind and spun Harry out near the beginning of PG.

[Bad Alias]

It isn't completely unsupported.  Somewhere there is a WOJ that says Harry and Molly looked for her after the fact.  But remove here from the story and the story falls apart.  She pulls the actors together, she sets the stage, she puts the idea of using fear as a cure, and so on and so on.  And then poof, gone like a fart in the wind.  We last see her taking Molly home.

I think there are a couple WoJ's about it. I recall one where someone lays out the case that Marling set up basically everything in the book, the convention, Madrigal being there, the convention being next to Pell's theater, gave Molly the idea of using fear. Then Jim gave a typical reply that implies approval of the theory but doesn't confirm it.

Jenny's dead in Prove Guilty. The "Billy's Wedding" story is between Dead Beat and Proven Guilty.

[morriswalters]

If Maeve was involved in any of this, it's something that pretty much can only be confirmed in a time travel Dresden story.  Yes, in theory there could come a time when Harry works out most of the details and in a conversation or dispute with Mab, she reveals her daughter was responsible for the attacks, but that wouldn't be very good writing.  It would be far better for Harry to go back in time and learn of Maeve's role that way.  Plus, many of us assume it was future Harry who who hit the blue beetle from behind and spun Harry out near the beginning of PG.

By Cold Days everyone working against Mab is dead.  Excluding any unknown members of the Black Council.  Meave, Lily, Thorned Namshiel, Slate and the Reds. 

Lily implies that Maeve sent the fetches.

“No, Murph,” I said. “They planned for that. This whole raid was a setup from the get go.” I jerked my head at Lily. “Wasn’t it.”
“That is one way to describe it,” Lily said quietly. “I would not, myself, interpret it that way. I had no part in bringing the fetches here—but their presence and their capture of Lady Charity’s daughter presented us with an opportunity to temporarily neutralize the presence of Mab’s forces upon our borders.

Maeve is infected by this point.  We know this because it is at this point that Maeve starts selling the idea that Mab is Crazy.  This will be the central point of Clod Days, is Mab crazy? And then there is this.

“I think we got played.”
“By the Summer Lady?”
I shook my head. “I think Lily got suckered just as much as we did.”
He frowned and rubbed at his head with one palm. “How so?”
“That’s the part I can’t figure,” I said. “I think someone set Molly up to be a beacon for the fetches. And I’m damned sure that it was no accident that those fetches took Molly to Arctis Tor when it was so lightly defended. Someone wanted me there at Arctis Tor.”

The last line is the heart of my central thesis, this was about Harry.  Of course this is my head canon and as such may be terribly wrong.

As for who hit Harry this is answered in Cold Days indirectly.  Proven Guilty is probably the point where Ace starts shadowing Harry.  We can infer that he is shadowing Harry because he ends up using Lacuna as Harry uses Toot.  And she is almost Toots size. Something possible only if he had been watching Harry for a long time.

@Bad Alias
Thanks.

[Avernite]

My hypothesis that indeed the plan was always for Summer to get dumped into Winter's wellspring, and I think it is MAB who wanted that.

She probably had promised someone to 'order all her troops to stay at Winter's borders' which means she needed some way she didn't explicitly order to get them off.
Something like Summer's fire, which triggered responses beyond orders in all of Winter.

[morriswalters]

I don't know that Mab planned it, but she made it possible for it to happen.  I think that Mab's plan was to protect her future Knight.  How it played out told her who the players were, and what they were into.

It also foreshadows a number of things if you assume that Michael was being moved on the Chessboard.  That implies that Mab was playing with an Angel.  Molly made a choice, but Harry was innocent of any knowledge of what was happening in the background.  So Uriel, assuming it was him, could help Harry.  But he couldn't save Molly.  Which makes Uriel's gift to Harry in Small Favor more understandable.

[g33k]

I don't know that Mab planned it, but she made it possible for it to happen.  I think that Mab's plan was to protect her future Knight.  How it played out told her who the players were, and what they were into ...

I DO think Mab planned it; or, if you'd rather:  she set up some specific parameters (e.g. the apparently "weakened" Arctis Tor defenses) and then watched her enemies and frenemies (and putative allies) as everyone reacted, as plans unfolded.

I think this book is where Mab came to suspect Maeve; or maybe she already suspected, and this is where Mab proved that Maeve was compromised.  This is where Maeve made a bald-faced lie; maybe Mab knew then, or maybe she needed to investigate to see if Maeve could possibly have been tricked into believing the falsehood, so that it wasn't a "lie" as it left her lips.

And there at the end... with fresh Summerfire dumped into the Winterwell...  Mab broke cover, and winked at Harry Dresden.

She was up on the rooftop all along.

She saw the Fetch arrive ... with the daughter of a Knight of the Cross.  Yeah, Uriel is involved with THAT situation; not to mention that sometime about then, there's a Hellfire-fueled assault on Arctis Tor:  again, something of interest to Uriel!  So she's gotten Maeve to break cover, and she's unmasked a Denarian; no small accomplishments!

Then Harry comes along, and HE sees (smells) the Hellfire, so Harry knows.

And then Harry does his Harrying, and we know how THAT turned out (oddly minimal property damage, given Harry's usual record).

And then she winked.

A wink can be many things; but it is always about secrecy, privacy:  I know, and YOU know, but they don't know.  It is complicitous, an alliance (however limited) between the two.  It is communicating, and not acting:
 if this had offended Mab, she could have taken out Harry then and there (but she was busy shaping her next Knight).

Mab -- a faerie, to whom secrets are valuable -- gave away the secret of her presence there on the roof.

I return to my prior theory -- Mab wanted Harry to do this specific thing.
 Summerfire was just a distraction, though (likely an unpleasant one, but nothing Mab couldn't withstand).

What Mab got wasn't just Summerfire -- Mab (guardian of the Outer Gates) got the magic of a Starborn Wizard dumped fresh into the Winter Wellspring.

AND (bee tee dubs) she unmasked multiple key actors of the conspiracy against her.

You just don't get much more Win than Mab got that day.

[morriswalters]

I'm pretty sure Mab had Molly brought there. All the pieces had to be moved to their proper places and times.  Everything was designed to put all the players in the warehouse at the proper time to make sure that Harry didn't have to throw down with the Senior Council.

If you want to go full on tin hat, make Goodman Grey Sandra Marling and take his reticence at the Carpenter house in Skin Game as a call back to Proven Guilty, for which Uriel seems to be holding a bit of a grudge.