Like I said before, I think you're reading a bit too much into this. A little more than a bit, but not quite a lot.
What's important here is that beforehand he wasn't saying anything and minimizing his presence. He was lost in thought, and Artoriel putting him on the spot broke him out of it. I'd wager he's just trying to come to terms with everything that's happened (you'll remember he wasn't at the mission briefing for the raid to free the Vault from the True Brothers' control), as what we've seen of Emannellain suggests he's not very good at taking change and responsibility (see: skips class, drinks regularly, flirts with Lady Lainette instead of getting to the mission, gets captured by the Vundu due to carelessness, cowers behind us during the escape, etc).
There is likely some resentment towards Artoriel in there, but just why I can't guess right now. I don't think it has anything to do with Haurchefant, though.
The expression is a bit off, but I'd bet that's due to the camera angle and lighting. Personally, I think Artoriel was just smiling because he's looking forward to what the future holds now that he knows he's not alone. Even knowing betrayal and despair, I want to believe in people instead of being suspicious of everything everyone says and does.
Edmont's expression didn't change, and looking at someone when they talk isn't strange at all.
I think it's just character development on Artoriel's part. Realizing that, even though Haurchefant was his illegitimate half-brother, he was still more a knight than anyone else in House Fortemps, and now that he acknowledges and accepts that he wants to do right by people the same way his half-brother did.
There could be foul play somewhere in there, but there are endless possibilities, and I don't want to be suspicious of everything everyone says and does. (Or: everyone is possessed by an Ascian / in league with the Ascians.)
That does seem to be something they're building up to, and I hope to God it's done as well as it was in Fire Emblem: Awakening...