As for Chris Avellones influence on the writing: I don't think, that he will get to deep into the process (I think he left Obsidian, because he's a bit tired of writing for video games), but maybe he will act as kind of an editor for the other writers, to polish their scripts.
He didn't say why he left, but I speculate that it is because they cut 50% of his writing for PoE. He only wrote Durance and Grieving Mother and most of his content for them was cut. They were still better than everything else in the game even while unfinished. This isn't the first time he leaves a company.
