I think this could be an interesting idea. Imagine the fallout if Buffy and Angel slept together in an attempt to bring out Angelus (he has information the Scoobies need, a bit like in season 4 of ATS), but Angel didn't lose his soul!

I also think this could explain why Angel needed a shaman-induced fantasy in season 4. The illusion could force him to forget his fears of Angelus and the fact that the situation was forcing him to do this when he really didn't want to.

Hypothetically, yes. I believe that he could lose his soul that way because he loves her. It was proven in that flashback where the Shaman took his soul in ATS. They used Cordelia as the face and yet as soon as he felt the soul start slipping from him he murmured 'Buffy?'. He was legitimately startled and hardly thought it would happen with Cordelia. Or anyone else for that matter.

He'd never do it deliberately.

I thought one of the reasons they used a shaman/used an illusion in ATS is that the shaman could make Angel forget about the risks involved in losing his soul.

It'd be in a scenario like ATS s4, where they need Angelus to return because he has information Angel can't access, or possibly something like needing Angelus present to bind the soul permanently (something I read in a fanfic).