Jim has already answered those questions.1

Neither of those address the questions at hand explicitly enough, apparently.  [i/]I[/i] feel like the issue is pretty clear, but it hasn't stopped literally

years

of back and forth on the issue and it just keeps popping up.
   
He talks about how there's not really a king of summer/winter, but doesn't address the nature mantle of the Erlking specifically, he doesn't remark on whether it's a discrete thing in an of itself or if the Erlking is just a beefed up Sidhe.
I don't think that it is necessarily the same kind of thing, that it wouldn't just hop to another vessel.

In the second quote Jim starts at one place and ends at another. Like I said, I though the meaning was clear, but apparently it's not enough.

Someone just needs to ask "During the Darkhallow, would the Erlking have gotten sucked up and eaten along with all his power/mantle?" and "Do all mantle act like the Queens' mantle where they are independent things, that jump to a new vessel when the old one dies?".

In the DFverse Bob says something to the effect of there being skinwalkers like dudes who can shapeshift, and Skinwalkers the Real Deal. 
   
Lots of cultures have generalized shape shifters in their cultures, things that can morph into all manner of beasts and imitate other people. That's common enough. I don't think the term "Skinwalker" specifically comes up anywhere, but some one thinks any other culture in the world uses a similar term, I'd be willing to accept even loose translations.

Something that has been mentioned at least three time in the DF so far are the Rakshasa. These are similar to the Naagloshii, but distinct enough in their mythology to assume that they are entirely different creatures.
So there certainly are other Shapeshifting baddies in the DFverse, and we'll *definitely* see at least one more in the Rakshasa. It's been mentioned too many times to not be used.

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Regarding the earlier conversation about the various types of immortality, I'd like to point out the Super Ghouls in the Raith Deeps. They were all shot to pieces, and could reasonably be called "dead", but Dresden watched them get better.   
I'm pretty sure that that scene is pretty much the prime example of what happens when certain types of Immortal creatures get damaged, compared to little i immortal creatures. Just like death is a spectrum, so too must immortality be a spectrum.

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To answer the OP, I don't think there are very many things that can act like the Swords, or at least I hope not, because that would cheapen them. Nearly every culture has a legendary weapon or two though, and I don't mind that since *something* has to allow a vanilla human to be a heroic champion.

I'm of the opinion that the Athame might be powerful on it's own now, but that's not the most important thing. If Mab and Titania are Morgana/Morgause, or vice versa, then that would mean that the Athame is a powerful object that is tied to one of the Queens. So, given what we know about how personal connections can be used against a being, I'd think that Lea might have had something with a direct metaphysical line to Mab or Titania.

interestingly they look like Lovecraft outsiders, I think due to the common 'belief' that that's what things outside reality look like. Just like Fearbringer pulled his mask together from disparate things that cause fear. its hidden in how the fetches become something more than a mask, but the thing itself. Imo at one point when Jotenheim was what was considered beyond the known universe, outsiders were ice giants. 

 
   
I think the common belief is that the Ice Giants were to the Norse Gods what Summer is to Winter (or vice versa). 

It was only the Eldest Fetch that became more than the mask, something about him was more "real" than the other Fetches:

This thing was no fetch, no changer of form and image and illusion. There was no shadowy mask over an amorphous form, no glamour altering its appearance, which my salve would have enabled me to see through. This thing was a whole, independent creature. Unless maybe it was a fetch so old and strong that it could transform itself into the Scarecrow in truth and not simply in seeming.

   
   
In the DF, Lovecraft was writing about the actual Outsiders, so it's not that he influenced people, he was first inspired by them.
HWWBefore might have drawn on the collective human psyche to get form, but overall I think Outsiders look jacked up because, where things on the Inside have a form that follows function, the Outsiders just cobble together whatever hot mess will work for the time being to get to the murderizing.   
Like the Names of the Outsiders, and the Outer Gates themselves, the form people see just takes on whatever their mind can handle, so, you're probably at least partially right about them taking the form of whatever the viewer sees as fearful, but also it'd be whatever is most alien.   

its very obvious the Cottage on DR and the mothers are one and the same. When Harry joins MS on the rock, which is in the same spot as the lighthouse, it allows them to travel to the outergates. Evidence the sleepers and the dreamers(outsiders wanting in) are one and the same?

The Mothers are powerful enough to port around basically wherever they want. Even in Changes it seemed like Vadderung was powerful enough to make an item that opened a wormhole directly from Chichen Itza to Chicago. The Mothers can open a Way from their hut to, anywhere essentially. I don't think anyone can got to the Mother's place, or even find it, unless they want it to be found. I don't think Harry really just stumbled upon them in SK, more like they manipulated the NN so Harry would find them.

Knowing the Merlin, it *could* be that he was just that well-connected that he got a shortcut on his island to see them, but it would be by their Will. Now I could see DR having a shortcut to the Outer Gates on it, that would be awfully convenient.

Let's be real though, if there's any sure-fire Way to get to the Mother's place, it's to use a copy of *Who Framed Roger Rabbit*.

Not too long ago I was talking to someone about this. I was thinking that Mab is Morgana or Morgause, and one of her sisters is Titania. 
   
The Athame is supposed to be on par with the greatest of the Swords, but Jim said it wasn't so much what the Sword was, than who it belonged to. Just being Morgana's and being used in a ton of high-level magic might be enough, but that's some *serious* juice to have for just being passively charged, the Swords were made by a Cosmic event, and this athame was made just from just one Wizard lady, even if a crazy-powerful one? One would think that there'd be at least a few more things like that floating around if it was that simple. (Of course this is all hindered a bit by the presence of Nemesis, which might have made an unequal trade possible.)   
If Mab was Morgana though, then it all makes sense. Lea didn't want it just because it was powerful, she wanted it because it was Mab's or Titania's. It would mean that the blade has a strong personal connection to a current Queen, and *that* warrants it having serious power, and would explain why Lea having it caused such a big power imbalance that Mab felt she couldn't be allowed to keep it. 
   
That would put Mab as a contemporary (and possible apprentice/lover/foe) of the original Merlin (Hmm, Molly/Harry parallel? See: Mary Stewart), a sister to a Knight of the Cross, and she'd have been a legendary badass even before becoming Lady. The timeline fits since we know by WoJ that Nicodemus is older than Mab. 
Around Arthurian Times Mab becomes Lady, and probably around the Battle of Hastings becomes Queen. 
   
This is where it gets  muddy though. In *The Once and Future King* series, Morgana and Morgause get merged. It would seem that Jim drew from this at least a little, since one of the books is titled "The Queen of Air and Darkness".

There's lots of versions of Arthurian legend, so it depends a lot on where Jim chooses to draw most from, and it's likely that he'll pick and choose from various places as-needed, and put his own spin on it all.

Until proven otherwise, I'm solidly in the belief that Mab and Titania are Morgause and Morgana (Elaine, the third sister wasn't really a primary character). I was more thinking that Mab was Morgana until I read about the Once and Future King Series, after that, it seemed obvious (almost tooobvious though...).

Somewhat related, there's also this WoJ (emphasis mine):...   
   
I don't think Earth can be "relatively conflict free" unless it is  a universe where the Outsiders are not trying to get in.   One possible reason for that is because in those universes there is no mortal magic on Earth and so nothing can summon Outsiders.   So one possible ending for the BAT is that  Harry permanently shuts off the world from the Outside at the cost of ending all mortal magic.   

   
     
Jim's quote is saying that in some universes, the Earth is not the focal point of Cosmic  conflict. In the DFverse, the Outer Gates are located in the earth's portion of the NN, and it's Earth's Mortals that are the focus of Cosmic importance, while the aliens that live in the Andromeda galaxy are kicking back with some space lemonade wondering what's for dinner. 
     
In some parallel universe, there's a planet populated by hyper-intelligent creatures that look suspiciously like donkeys, who have been locked in a fierce battle with the Outsiders. Meanwhile the people of Earth in that universe are celebrating an unprecedented 1000 years of peace known as the "Ice Cream" age, which was only briefly broken by a scuffle due to a disagreement over which topping was most appropriate: fudge, or butterscotch? The disagreement was mounting toward war until the last second when a clever Wizard named Gary Cresden discovered people could actually just use both.   

Sure, but you'd still have to explain why there are no mortals on that Earth summoning Outsiders.

Because they don't know about them. In some universes the Outsiders never get that far, and when/if they do, the Earth isn't the primary entryway. Maybe sometimes they get that far and the Earth becomes more important. Maybe sometimes there's no intelligent life on Earth.

Thing is, I don't think we've ever see any angelic forces fighting directly against the Outsiders.  I mean (as far as I remember), you've got Michael at the end of PG, Nicodemus' "blinking" when he hears about Hellfire at AT (and some of the stuff he says in SG) and (if you buy into the theory) Rafael-Demonreach. 
 
Now granted the whole Outsider threat was only explicitly stated in the last few books so its not impossible that we haven't seen the whole picture, but given that a whole angel is given the sole job to protect the possibly dying Forthill, you'd think Harry would have run across a few hints of their existence at the continual war at the Gates. 
 
I guess it I just haven't have gotten "that vibe" so far, though of course it might be Harry's misunderstanding of the situation.

 
   
The battle at the Outer Gates is only one part of a war that spans infinity. The Outer Gates are one portion that kind of acts like the front door to the Outside. We see the Outsiders attacking the Gates, but, if there are Gates, doesn't that imply that there are "walls" that wrap around everything else? Why not try to burrow through the walls?
We don't know the whole nature of the Outside, the Outsiders, or the War itself. It could very well be that while the Winter Sidhe are fighting things that they can actually beat, the Angels are keeping things at bay that the Sidhe are no match for, or it could be that they guard the walls of infinity themselves from Outside, and the things getting to the Sidhe are only the things that slip by.     
Since there is an Inside, one would assume that whatever deity created it (assuming it was actually created as such), would want to keep it in order. There are supposedly an innumerable amount of angels, so having a handful watch over the dead isn't that strange. Even in the midst of war, aspects of regular life still go on.
     
Also, regarding the angels and the daily squabbles Inside. In some Gnostic lore, the universe as we know it was made by a demiurge, an imperfect god-like being (usually with another, unreachable-to-us god above this being). The demiurge created the universe out of Chaos, and since the universe is made from imperfect material, it is inherently flawed and prone to darkness and chaos. Assuming the WG is a Gnostic demiurge, the conflict with Lucifer, and all the problems of the universe makes perfect sense. In this scenario, the Dresdenverse isn't just something separate from the Outside, it's *made from* the material of the Outside, the chaos that the Outsiders want to bring *is already here*, part of everything. So while the WG was able to make so many good things, conflict was inevitable, even among his own crew. The goal then would be to manage the chaos Inside as well as Out.   
Moreover, Freewill is in essence an element of Chaos inside an ordered system. A perfectly ordered and predictable system couldn't have Free Will. The creatures that battle Free Will want perfect order (and thus possibly safety from the Outsiders). Free Will both poses a threat to existence, and at the same time there's no point to existence without it.

edit: Forgot to address the base concept of the thread:
   
Wasn't it said in CD that Victor Sells and the FBI people were infected? I know it came from a suspect source, but it wouldn't really make sense to throw that into the story without it being true, even as a misdirection to throw Harry off.
   
I think there's just generally better targets. We don't know what Nemsis' limits are, it's been hypothesized that there might be a limit as to how many people it can control, or some other limiting factor that makes it infeasible to just spread like the flu. If it was easy as catching a cold, there'd be no stopping it, so there must be something more to getting infected by it.

Even if people have Free Will, they still have an inherent nature, it's just that most people never really make many significant choices to go against their nature. I think there's a WoJ about that. A Nemesis infection would turn someone away from who they are, hence Lily's bit about a loving father turning to consume his family, and people sworn to the uphold the law turning to vigilantism and murder. Sounds pretty Nemesissy to me, and the sludge that was all over that guy's Soulgaze seems to me like perhaps the earliest foreshadowing of Nemesis. 

I'm not too sure about that. A mantle is something that contains power and if a wizard is strong enough to be considered  for the position of Merlin, they wouldn't need that power. I think we are putting to much importance on the existance of mantles. It would be kinda of cheap if every power up Jim gives to his characters was mantle based. If Harry isn't the Merlin, he wouldn't gain the Merlins power through a mantle. It would be more realistic if Harry obtains Merlins journels and learns from those

Lol sorry if that confused you. Power in the form of a mantle is inherited, while power from a title is earned. THE Merlin of the White Council earns his power through training its not given to him from a mantle. Otherwise a regular human could be given the mantle and become super powerful without the training and discipline they would require to control it

There are Mantles and mantles. The Fae Knight/Queen Mantles are magical constructs which seem to be discrete entities from their hosts. These can be passed around and grant overt magical powers. Little "m" mantles are a little more mundane - one could don a mantle of authority for example - there is power in the position/title itself rather than giving an actual magical or physical power boost.

The title of Merlin probably doesn't come with a magical power boost but it definitely comes with the power of authority and political weight. The position might also come with secrets, special access or special contacts not privy to others - that is power in a different form.
The position of Warden is a little more ambiguous. Ebenezer already referred to the position as a mantle, but I think he meant it as people normally do - a position with special rights and responsibilities. The mantle of Warden does seem to come with its own power boost in the way of limited intellectus, but its real "power" is in giving Harry control over the prison and Demon Reach. He's been granted authority rather than a direct power boost.

So yeah, when people talk about Mantles or mantles I think we need to clarify what we mean: if we are regarding a specific role as being a Magical construct with some cosmic purpose, or a more mundane position. It's important because higher powered beings in DF have consistently been in or previously held positions of authority and/or responsibility. It's almost as if they are inseparable past a certain point, so Harry WILL most likely gain extra responsibilities for every power-up he gets but not every mantle in DF is a magical contsruct (that would be silly).   

Also, it's been said over and over and over, but power does not immediately or necessarily mean quantifiable magical energy units. Power fundamentally is the ability to act. There is physical power, magical power (in seemingly quantifiable units), there is knowledge, there is wisdom, there is authority, and probably many other forms of power.
In the books we see that Harry has much more metaphysical mass than most Wizards, yet others who don't have as much are far more powerful than Harry is because they have authority, knowledge, and wisdom. For example, Luccio is able to do more powerful fire spells than Harry with less energy because she has finer control. Same thing with the Archive, she has crazy amounts of knowledge that allows her to wield magic in ways Harry probably couldn't imagine. Harry could beat Fix butt naked because of his absolute knowledge of his surroundings. However, Marcone is more powerful than all of these people in that he can hire the best hit-men in the world to do his bidding since after all, money is power.

If any corporeal person was Gollum it was Martin or Susan. Martin was the pitiful creature that lead them to Mt. Doom and it was him dieing that allowed the battle to be won. I guess that would mean Susan turned into the ring at the last moment. Or Susan was Gollum since Gollum's death was kind of a predetermined sacrifice that Gandalf (Lea) foresaw.   
Of course it's never going to be a perfect analogy.