You only lose them for the rest of the quest. I'm not sure if there are members you can bring that will resist the temptation. I had Anders, Aveline, and Varrik, and lost Aveline and Varrik. Anders was also possessed by Justice for the whole mission which was kind of neat (but I got 5 Rivalry points for bringing him).
Experts are to Fox News what winners are to the Special Olympics. If you show up, you are one.
You can't take Sebastian. If he's in your party he'll say he wouldn't find it right to enter the Fade as a man of the cloth. He feels its foolish or something.
I sided with Torpor and chose "power." I went through both sides and let them finish by not saying anything to either demon (not warning Feynriel that it was a trick). This is what Torpor told me to do. Then I brought Feynriel to Torpor and the demon was ready to possess him, then he was like this mind is shattered and attacked me. Even though I TRIED to help him. I'm not sure if I messed up and it is possible to get something out of him or if this happens no matter what you do.