Because everyone knew that she was becoming more paranoid during Act 3, Anders knew what would happen when he did that
He deserves as much as the blame as Meredith does
Yes, yes he does. As I've stated before, I liked Anders more as a person. It helps a lot that I had a chance to actually get to know him. Had I had the same chances for Meredith (or Orsino,) I might feel differently. Currently, one sympathetic story about her past doesn't condone her actions, which I've seen an excuse people make for her behavior. In my eyes, Anders upbringing also doesn't condone his actions towards the Chantry. They were both in the wrong with their actions, both had valid points, and yet were needed as catalysts to drive Thedas into the future.
This, I think, is why dead or alive he needs to stay out of Inquisition. I don't see how his story can continue after that. Plus, it's probably rather tricky to give a cameo for a character that is supposed to be dead. Mind you: Anders was possibly retconned (I kept him alive in Awakening,) Leliana might have been "killed" near the Sacred Urn (also a retcon,) and Zevran was a bug. I suppose Wynne in Awakening would count, but it was a very small conversation. You didn't even turn the quest back to her.
If Anders shows up, that's about the most I'm expecting: A five minute cameo that would leave Anders fans unsatisfying and those that hate Anders decrying "fan service." It seems like a lose-lose situation either way. Probably best to keep him to a codex and some mentions of him, and leave him directly out of DA:I. While there might be some wrapping up of loose ends in future EU, I think Anders' major role in the overarching story is done.