I happened to be re-reading Changes this week, and it turns out that the Fae can "lull [a Red Court vampire] predator spirit to sleep." The Leanansidhe claims to have done that with the "vampire part" of Susan and Martin when she rendered them unconscious.  As you'd expect in the context, there's no discussion of whether the ability also extends to White Court demons but Lea speaks broadly of vampires.

My money is on the White Court demon being unable to affect Harry as the WK, Mab giving Harry a charm that lulls a WC demon to sleep, or something to the same effect.  In such a case, with the WC demon's effects canceled, there would be no reason for Lara to blister (regardless of whether Harry's love for Murphy would still protect him from a WC vampire with her dead) and consummation of the marriage will occur as Mab likely requires.

So we should assume that Mouse now knows Cowl's scent, and that Cowl isn't anyone that Mouse has met before else he would have recognized Cowl in this story.  That should narrow the list of suspects.

Apropos to maybe nothing, but has Mouse met the Gatekeeper?

Release Thomas but keep imprisoned his demon, the one that makes him a White Court vampire?  That would effectively make Thomas human again.  I believe there's a hint somewhere in canon that separation is possible.  Why not DR as the instrument?

That would make for a nifty but conflict-laden solution to Thomas' status/prognosis.  At the end of Battleground, it's stated that Thomas is on the verge of death and wouldn't survive being released from DR unless he can immediately feed on a human to the death.  But I can't see Harry letting that happen even if Harry hated the human in question, but such a release and feeding undoubtedly will be what his 'intended'/'wife' will demand. 

But if DR can separate Thomas from his demon, Thomas isn't available to make that call (basically because he's unconscious unlike other DR inmates).  Would Harry take on responsibility for making that call on his own?  Lara would be vehemently opposed.  More conflict.

Also, can DR separate the One Who Walks Besides/Nemesis/Nemesis infection from its host like Justine?  And DR keep OWWB/N/N infection as a prisoner?

BTW, not clear to me at all how the OWWB/N/N infection works.  I can't believe the OWWB is wholly encompassed within Justine (and  was once wholly encompassed within Harry's godmother), and that others can't be simultaneously infected. 

No skipping to the end, but it's past time for Jim to get a co-author (or more) who can crank out a book every 12 or 18 months, with Butcher providing the outline, direction, and editing.  As a fellow human, I certainly sympathize with Butcher's RL events; many of those events are very painful and some are time-consuming.  But his events are the type of things we each go through, yet have to continue to function in our jobs and in other aspects of our personal lives.  I've read many an author explain their process - I get up in morning, I sit down, and I write XXX words.  Viola, a year later, the novel is finished.  Butcher apparently isn't of that persuasion so he needs co-author(s) who can write on a deadline regardless of what RL throws at them.

And you can't tell me that Butcher's publisher hasn't, isn't, and/or won't be pushing this approach.  Speculating, but I'd bet the publisher won the fight over splitting Peace Talks into two books because they like money too, Butcher hadn't been pushing out any new product, and they had to be concerned about a shrinking number of fans who were still willing to pluck down hard-earned $$ for a book after many years of waiting.  (Collections of short stories don't count, BTW.) 

As a customer, I expect an end to the series.  I thought I'd live to see that but those odds grow longer each day nothing is published.  For that reason, I certainly would never recommend anyone start this series - or any other with an overarching story - until it's finished.  And I'm sorry I recommended Dresden to a friend 15+ years ago based upon Butcher's early pace, since now there's no chance she'll be alive when the series ends.

I also thought Eb was uncharacteristic.  Maybe infected?

Given the lack of any actual sounds of combat or violence, I presume the entity was admitted by the Svartalves, rather than forcing their way in.

The entity you reference wouldn't be daunted by the Harry's threshold, or WK Mantle, or anything else Harry has on tap.

Nevertheless, I like your theory; that could indeed be who it is!  I'm a bit perturbed, though, by trying to envision what might bring that one to Harry's door; I'd expect an emissary to come deliver an invitation too "good" to refuse.

Struck me that if the entity at the door had been let in by the Svartalves or they could have or were permitted to alert Harry, then they would have let Harry know he's getting a visitor given how organized and efficient they are.  That, and the fact that the alarm went off (I assume that if the entity had been cleared, then there would have been no alarm), makes me think the entity is uninvited.  I don't think there was a battle - as you say, there would have been no point.  I don't think the Svartalves would engage in a futile gesture; too rational for that.

Knocking makes all kinds of sense for my guess - the entity must know about the Old Ones, the Outsiders, and definitely Nemesis, and you don't unnecessarily antagonize those who are objectively your allies, and particularly Mab and the Winter Court in light of the role she and they fulfill.  (Assumes the entity likes reality as it is, which may be a big assumption.)

It strikes me that the entity is very much a solo act, and wouldn't act through emissaries.  An 800# gorilla who doesn't need them, goes where the entity decides.

As to why the entity is there - maybe the entity lives in the same complex and just needs a cup of sugar?