@TKS I partially agree with you here. I am in agreement that Anders should not have been allowed to escape that many times and should have probably been executed. However, there is no way in hell that anyone could have seen Anders eventually blowing up the Chantry in Kirkwall.
One cannot predict all the events that it took to get to that point: Finding himself in Vigil's Keep, becoming a Warden, meeting Justice, fusing with Justice, going to Kirkwall to save Karl, staying in Kirkwall, having Justice's ideals slowly take him over and years later blowing up the Chantry.
This is getting a little off-topic. If Anders is dead/on the run, he isn't in Kirkwall. As a result, I think that Sebastian returning doesn't have much of a point. I could be wrong, but I don't see him with much (if any) future screen time. We might see Seb again in a future EU, but I'm not banking on his appearance in Inquisition.