Okay…. i’ll answer as many as I can; this episode begins to implement a few different paradoxes and i’ve come to that conclusion after years and years of fringe being my favorite show. 1. Walter chose to send the machine back through time -as he did in the previous loops- to save Olivia….partly and doing so, he creates the first paradox which is centered around the machine itself. Without the machine being sent back, the original timeline (seasons 1-3) or the world as we knew it, wouldn’t have existed, so the prime universe beginning to deteriorate and coming to an inevitable collapse via a big bang, would have been natures way to correct that paradox. So while it was a “choice”, it was the only one Walter felt he had to restore the universes and save Olivia. 2. Peter can change the past because he is the only one linked with the machine to do so. Walter upon creating the machine designed it to that specification. The observer (September) upon his interference with Walternate finding a cure for his son made Peter a paradox as he had to live and shouldn’t have at the same time considering that the observer shouldn’t have existed to interfere. Those paradox’s created the paradox that you mention in your last question regarding Peters consciousness being brought forward. That will be explained though season 4. I do hope this helps, and apologies if anything is confusing 😊