I don't trust Feliar because there's a lot of interactions with him that ask if we trust him or not, definetively more than any other character.
You are not asked this when you met Ste or Korg (maybe one time, but it's just that). Feliar is very persuasive and he's actually very close with the leader of Northcrest army, Everett. Since we know the shapeshifters are, actually, pretty interested in Northcrest government, this is something we should consider.
Anyway, I really hope the shapeshifter revelation to be a big and main plot. It would be hell good to see a NPC I loved and trusted has been betraing us this whole time.