So far as we know, all mantles are constructs, made by an entity for a purpose.
Who could (and would!) create such a mantle?

I suppose the Archive has the knowledge to create it; and there's enough power on the Island to do the implementation... but, why would she?  Her Oblivion-War duties are big on secrecy, and adding a new & independent globe-trotting Mantle to the mix looks like a huge gaping OpSec hole...

The Gatekeeper sent Harry that cryptic "black magic in Chicago" message; so he seems to already be capable of doing "black magic scans" & the like (though  that could have just been smoke & mirrors, with Mab tipping off the Gatekeeper to feed Harry the bait, so she could reel him in...) .
 
Also, Harry "I've got Authority Issues" Dresden would himself be a really ironic target for becoming the ultimate Authority Figure on the issue of Magic!  (this is not, I should note, any argument that Jim wouldn't inflict such a thing upon Harry!)

Lara may be anxious about Thomas, but she won't blame Harry for his condition. She's very practical, does love her family and realizes that not only did Harry not cause this injury to Thomas, but that she forced Harry into considerable danger in saving Thomas' life from the Svartalves and hiding him on the island.

I also think Thomas can remain a vampire and still wield a Sword. We've already seen Susan use one on a mission of love to save her daughter. Thomas can use the same Sword of Love to save the souls of Justine and his child. What happens after that is open. Perhaps prolonged exposure to the Sword will change the nature of his demon. Or maybe he's only a Knight for that mission since we've been told that being a long-term sword wielder is not the normal. Maybe they get rid of Papa Raith and Thomas becomes the new White King and Lara becomes Harry's sidekick.

I'm sure all these guesses are wrong but until Jim tells the story, WAGs are all we've got.

  Hasn't Harry already come back?  At the end of Changes when he was shot, he was mostly dead, very close to all dead.. He was already Winter Knight when he took that bullet through the heart, would have died then and there on the deck, but fell into the very cold lake before he bled out and into Mab's arms. Hmmm... Wonder if Uriel had anything to do with him stumbling over the side into the water after he was shot?

Yes, I agree, Uriel has had a lot to do with preparing Harry for what is to come.  That was the whole point of the risky soul walk about, and Harry came back with a better understanding of what soul fire really is as we saw in Cold Days.  He also learned how to keep the darker impulses of the Winter Knight mantle under control.  However the White Council doesn't know any of this, they are asking the same question you are asking, except not what will come back with him if he dies again, but what came back with him this last time when he "died."

I think Mab's reasons for wanting Molly dead if she dies are a bit different.  Mab is a control freak, and she feels that Molly isn't on the same page as she is.