1. I don't think going after Devotion and Dominion necessarily helped Odium find Ambition, he probably ran into them, or gave up on his search. I made a theory a while back about Odium's decision to Splinter Devotion, I think it is likely that Devotion and Odium are a Shardic pairing/opposite, one involving love, and one involving hate. If this is true, Devotion was probably number two on his hit list, so he probably figured he'd take care of them while waiting to hear from Ambition.
2. I remember a WoB about that, but I can't remember what it says. I think it may have been possible for a Vessel to survive afterward, but in weird ways. In part, their Cognitive Shadow sticks around, like the Stormfather/Tanavast. I imagine that's what's happening on Sel, and wherever Ambition was Splintered as well.