...  But I've always seen Mab ultimately as an amoral force for good ...

Say rather, Mab is an amoral force fighting for the survival of Creation.
Mab would not do anything only "because it's good" -- only to advance her singular cause.

Now... that cause is, in itself, "good" to be sure!  But Mab ... kind of doesn't care that's it's "good."  It's just what she does, who she is:  Queen of Winter, Defender of Creation against the Outsiders.  Other "good" things are ... largely irrelevant.

I'm pretty sure she's an "end justifies the means" kind of girlie; she will readily do "evil" to advance her cause (unless she thinks there's a "Big Picture" reason that evil-doing would harm her cause).  She would cheerfully stretch every one of Molly's siblings across her Stone Table and bleed them out one by one, if it served her cause better than their continued survival (I cannot imagine how it would do so).

... Granted, I'd much rather see the Redcap sent to the Outergates to fight than let to run loose on the Earth ...

I'm betting the Redcap exists in exactly the same "uncomfortable for us, but exists where he does for the protection of Creation" space that Mab herself does.

... or perhaps Strauss (one of them).

I don't think Strauss was Austrian...?
And certainly didn't "die young."

</nitpick>

Also, @Mira is 100% correct to note that there were innumerable "Austrian Composers" and it doesn't have to be one of the "Big Names."

Nevertheless, I think we should presume that it was indeed one of those Names (likely Mozart):
  (a) Jim clearly has someone particular in mind, and it's doubtful he'd delve into classical-music-minutiae to unearth  J. Random Niemand.  &:
  (b) Mozart isn't just "an" Austrian composer, he's widely held to be the greatest composer ever, with a deeper understanding of music, and grasp of "perfect" musical forms, than any other.  I myself am far too plebian to judge (or even understand) such claims; but that seems to be a common report by "people who know."

...
Did she pick Lea?  Or did Mab pick her for Margaret after the fact?

Yes, Mab picked Lea long ago.
Lea was Lady Mab's version of Maeve's Jenny Greenteeth, per WoJ:  Mab's Handmaiden from before Mab was even Queen.

... A bargain that will really piss Harry off when he finds out what it is all about, namely with Lea.

We don't know the details (specifically, that it was a "bargain," nor that Mab was explicitly involved).  Only that Harry will be really, really pissed off at Lea when/if he finds out.  Jim dropped only the one minor nugget, AFAIK (I'm happy enough to be mistaken, if there's more info out there!).

... Just a passing thought, but it may be that Margaret signed up her fetus, the future Harry Dresden to become Mab's Knight in return for his protection, since as a star born he would be a target.  So free will is a big deal right?  If this is true, Harry never ever had a choice, he would be set up over and over again until he agreed to become Mab's Knight. His choice, free will, right, well, yes, and no, that point gets fuzzy ...

The thing is:  Maggie need not have made that deal.  Mab could still keep adding pressure to Harry, manipulating his situation without touching him, offering him power, cutting-off his other options.  She is free to advance Winter's interests as best she knows how, and Harry declining to take one lure doesn't prevent her from dangling another, and another... Every choice remains Harry's, it's still "free will."

I think we know Margaret didn't make a "deal" for Harry to become Mab's, because Mab didn't just take Harry -- she needed Harry to agree.

Oh I think it was more than that, for the simple reason we have on more than one occasion one Senior Council member saying to another with a wink and a nod about knowing what Harry was supposed to be.

Wizards -- especially wizards as strong as the Senior Council -- don't share well... especially they don't share their secrets well.  WoJ says they have all the pieces of the puzzle, but it's all split up in different hands and they aren't sharing.

It's precisely because of that wink-and-nod that I think they don't all know:  you don't wink-and-nod that way unless there's multiple layers of meaning, insiders who know and outsiders who don't; or layers of meaning, with progressively more-or-less informed.

Maybe some of them think of Harry as a violent borderline-sociopath proto-warlock, "meant to be" DuMorne's merciless enforcer (this is, I think, the "Official Story" given to the rank-and-file).  Maybe some of them think of Harry as "meant to be" a Destroyer (whatever that is, presumably an overpowered agent of chaos&destruction with some sort of fate/destiny thing pushing them on); this is the "secret" given out to some, who realize there is some further secret.  Maybe some of them think of Harry as "meant to be" a full-on Starborn, summoning & compelling Outsiders who outclass even White Council wizards; this is the kind of dangerous info that they don't give out -- you learn it when you figure it out for yourself.

Everyone winks and nods and moves on, but some are more-informed than others.

...  For starters, how is it that so many know Harry was star born to begin with?  Lot's of babies born world wide at the same time Harry was, but that doesn't make them all star born ...

I don't think we know; but I suspect that it's only wizards who can be Starborn; or at least, they're the most-meaningful ones.  A muggle who's resistant to Outsiders still can't banish them or or those other useful magical bits; just personally resist.

I think some of the older wizards have sufficient "starborn" info to figure out that Harry is (or may be) one.  I presume Margaret gave birth to Harry in a semi-normal setting (doctor in attendance & birth-certificate & all that).  So Margaret's son would be known to be born at the right time (once they knew to look for him).  But "that son of a bitch" DuMorne somehow tracked him down first; and after a few years in Justin's hands, they couldn't be 100% certain what was hidden inside Harry.

... Oh I won't argue with you on that, I think it's been established that wizards live maybe 350 years give or take if all goes right, I wouldn't be shocked if Mab was closer to a thousand years old.  The only possible exception in my mind would be Rashid.  I think he has a thing going with the Mothers, they gave him his eye a very, very, long time ago, and his background is more complicated than you think that's why he is fit to guard the Gates.

Yeah, Rashid is much older.  WoJ places Rashid as being a potent wizard around 735AD, taking down a powerful Outsider-summoning wizard (Jim explicitly takes from HPLovecraft's Cthulhu Mythos; HPL's fiction was clued-in, like Stoker's Dracula, though AFAIK we don't know who clued Lovecraft in, or why they did so).

I recently saw a theory that Rashid is one of the old Gods, like Odin.  One thought he *was* Odin (just a different Mask) because of Rashid's eye-thing; another pointed out Rashid being maybe Egyptian, and could maybe be the god Horus, who looses one eye (but it's later restored to him).  Also in the Egyptian pantheon, we have Anubis who's explicitly a gatekeeper (one of the god's formal titles).  So... lots of potential links, there!

But there's a crapton of time-dilation to be had in the deep Nevernever -- where the Gatekeeer perforce must go -- so Rashid could be legit mortal, but spends so much time in wonky 10:1 timeflow that his aging just doesn't match up with even other wizards.

... I don't argue with most of that either, Margaret also knew that, and knew how the White Council, maybe Mab, and or even Lord Raith wanted to use her as a brood mare for a star born babe.  However Margaret turned the tables on all of them when she selected Malcolm to sire her star born babe.

I doubt she saw through Mab's machinations; that's tough to do!  If she had -- and thought Mab was wrong -- then she'd hardly have picked Mab's Handmaiden to be Harry's FG!  Since she was inside Raith's own household, though, I think it very likely she managed to find info that he (and/or his allies) didn't want her to have; and of course she was already suspicious of White Council shenanigans, and likely could have seen through some of that plotting (particularly given Rashid's testimony that he and Margaret sometimes met on a friendly basis, as fellow Waywalkers).

... Possible, but I don't think she was the only one, I think the White Council may have also been in on it.  They seem to know what Harry is, or what he is supposed to be...

I don't think it's "the White Council" or even "the Senior Council" (as a whole).  Different individuals / groups within do IMO have insight into the Starborn/Outsider/Apolcalypse situation... and even have plots afoot!

I put Mab's plans as the deepest-laid.  She's likely as old as the White Council itself, older than any members, and she has the kind of constancy no mortal can match, the singularity of vision no group can match.  The plans that she set in motion, when she did so, were likely only noticed by Odin (and Uriel of course).  Everyone else entered the game later, and is playing catch-up.

I put the Black-Council plots next:  I look again to that infamous meeting, where Margaret invited Ebenezer to dinner at Papa Raith's (and Duchess Arianna was there, saw them "fighting like family"):  Margaret was trying to recruit Ebenezer into some sort of plot.  I think this was recruiting for a "Black Council" initiative, and specifically a Starbabe plot.

I think the WC became aware of this plot... possibly Ebenezar even reported to the SC or Luccio or other elements of the WC that there was a plot to create a starborn (I presume he carefully didn't implicate his daughter in his reportage).  With that awareness, I expect their counter-plan was to swoop-in at the last minute to steal the starbabe before the Bad Guys^(tm) could corrupt the infant.  Recall this is the sort of thing Bob has called "Classic White Council doctrine" (to use the bad guys' mojo against them).

... That, hasn't been spelled out yet other than to say, "a destroyer."  Of what, whom, or why? No clue.  Based on what we've seen of Drakul and Listens, we can guess...

My theory:  a "Destroyer" is any Starborn-gone-bad:  selfish, power-hungry, gone-warlock, colluding with Outsiders, etc.  In my theory, "Destroyers" are better-known than "Starborn" are (you'd think an old campaigner like Donald Morgan would at least mention the potential for a Starborn to be an asset, but in Journal we only see his worry & doubt about the Destroyer-potential; I conclude he's ignorant of the larger "Starborn issue;" and if Morgan's ignorant then I suspect most of the White Council is, too).

... With Malcolm, Margaret threw a monkey wrench into whatever plans they had, because Harry didn't turn out quite as planned ...

I think Malcolm was Uriel's monkey-wrench.
I think Uriel turned Harry from the WK that Mab schemed-for and wanted into the WK that Mab (and Creation) needs.

Yes, but was Mab in on that?

It seems the most likely.
It's Mab who is the long-term planner, the strategist, the master-manipulator.  She lays generational plans.

She wants a powerful wizard, and a Starborn, as her Winter Knight (the only likely-better option would be a Winter Knight of the Cross, but I suspect she realizes KotC's & their divine-guidance are beyond her ability to strategize-around).  She does not want this left to chance, needing to pick the best of the randomly-available options.

I think Mab (often (mostly? always?) working through Handmaiden Lea) chose Margaret as a Salic-Law-likely mother for a strong wizard, but one isolated from the WC and therefore likely to need to "make deals" to achieve safety for her child.  With subtle lures, Mab set Margaret on her course well over a decade before Harry was born, and "arranged matters" such that Margaret would seek out the FG bargain with Lea.  I absolutely think that was Mab's plan all along.

(n.b. I suspect Margaret & child weren't the only such plans that Mab had in hand; I wonder at the paucity of info about the Mallory's, Elaine's parents...  I also expect some other Mab-engineered Starborn-WK candidates, who've never appeared onscreen.  Because one of Mab's planning hallmarks is not to put all her eggs in one basket; she has backups to the backups for her backups, umpteen layers deep)