Re: Starting Dorian's Companion Quest
Ok after realizing I had a couple of autosaves left from just before you get to side with the mages, I went back (sacraficing about 15 hours of gameplay) and played through it again to see if I could get Dorian's quests to trigger.
I completely ignored him and quickly sped through to Skyhold without doing any more sidequests than necessary and talked to him the second we arrived. I also picked "Corypheus must be stopped." when becoming the inquisitor. After talking to him once (he has one cutscene about Corypheus) I left and came back immidiately, then talked to him again and the Felix cutscene triggered properly.
It seems the only way to fix this is to just not talk to Dorian at all. His dialouge options are all the same from when he was in Haven, and I guess if you exhaust all those options for some reason that breaks the quest. You also have to talk to him asap once you get to Skyhold or I don't think it will trigger properly. If you keep going with the story before then, I'm pretty sure it messes the whole thing up and makes it impossible to trigger forever after that point.