I'm not sure what your imagining here. A Presence Excellency and (optionally) Irresistible Salesmen Spirit shouldn't be detectable from the outside and can subvert Hotsuin just as easily as anyone else.
Oh boy. I just
have to respond to this, even if it's a little delayed.
Mental influence. Even natural mental influence, even just best-there-ever-was levels of Presence instead of outright superhuman levels... would be spotted. Certainly, being good at this also means you can make it less obvious to the target; just as certainly, anyone who talks to the target (without you being there) can easily notice that something is up. That sort of skill gets
noticed, and often gets you in the history books.
It isn't a complete OCP even in real life; most of us have had the experience of talking to someone who is better at arguing than you, and who's been able to address every flaw and convincingly argue that you should change your mind... despite their position being
wrong, and you knowing it is, you just aren't quite able to explain how. Skill at argumentation is a separate thing from
actually being right, a fact which I hope most people are aware of.
How much more important would that understanding be, then, in a world in which beings like Amu or Nyarlathotep exist?
=====
Some of you may be caught up to the Laundry Files. If you have, then you'll know what I mean when I say this: A socially-specced Solar is very similar to the Mandate.