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?
