If Peter was really meant to have died and because of him other people who weren’t supposed to die, did, I have no doubt he’ll want to sacrifice himself to save one or both worlds. So he too will let go of everything: his life, Walter, Olivia; but if we come to this point, I believe we’ll have the “there’s to be another way” and that’s when they could reunite, not only Peter and Olivia, but also Walter, Peter and Olivia, each of them offering their very own contribution. I never forget Bell’s words, when he told Olivia she’ll need Peter by her side, so I believe together they (and Walter, Walter is always part of the equation) will find that other way and be together. Of course, it can also happen that they’ll reunite in their hearts, but not physically. I’m only talking about the emotional aspects of their journeys since I know nothing about science, so it’s difficult for me to speculate.

And yes, I believe they have no problem making Olivia lose everyone she loves/loved. Just look back to all the things that happened to her. And she’ll feel and live Peter’s drama; probably the worst is yet to come. At this point things don’t look good for any of them, but Fringe is not a show about good and bad; it’s a show about the decisions we all make and that will affect a universe of people.