Please correct me if I'm wrong...
So all stayed the same to the point of the invasion by the Observers--but without the Observers being there to help Walter. Somehow Walter gets and saves Peter without Peter drowning, all on his own. And with this change, Olivia, Peter, and Etta are safe--but it looks like Peter remembers everything.
And, in the new timeline, Walter exists until 2015, when he suddenly disappears from this timeline.
And Michael is outside all timelines, since the one in which he was created never existed, but he was outside of it before it was changed.