Given that demons can "devour" souls, is it possible for them to keep the mind and self-awareness of the soul intact within them? And then, perhaps, use that when creating an aspect, so that they basically "recreate" the person for as long as they wish? In other words, an aspect that not only thinks it's mortal, but that thinks and acts as if it was a specific person the demon devoured?

I've thought about that option... I'd say that's probably a reasonable, if unusual, approach toward possession. I guess that means it could be something they can do, but don't quite as often.

(I have played that out in RP, though, so it has precedence. :) )

yesss :D another awsome explination

This is all very elaborated and cleverly and consistently thought through, and I can not see and logic flaws.

As a reader, I prefer to stick to what the stories tell though, as every great myth or story can be dispelled by too much explanation. I love Gil's storyline and never felt it required any extra explanation, let alone justification to why things happen, and it feels more interesting to learn little bits on the way. So I'd rather not comment on whether or not I think something in the background is cool or not so cool. :)