Okay so my answer is frustratingly, I'm sure, yes and no. Angel can try and teach runes/charms to prevent possession, but they'd probably have like a 25% success rate so that way it shows that she has her ability, but also the not canon stuff. As for pulling the demon from the body, I think that I'm gonna have to say no on that, because people can only be possessed once per character, and I want them to have the option to play to that with the fullest without having that as an out.

As for giving it form, can you explain what you're thinking here a little more please? I think I may be missing something, because the demons have form now, but if you've got a cool idea I'd definitely love to hear it!