Trying to explain time travel paradoxes in any form of media is an attempt in futility and a pointless endeavor. Every series will make its own rules as to how those paradoxes apply, every series will choose some answer for the purpose of reaching the story they want to tell, not necessarily what makes the "most sense".
Just relax and soak it in, don't try to analyze everything. Over-analysis is what killed Lost, The X-Files, Alias, and so many other amazing shows because people got way too wrapped up in meaningless minutiae instead of things that actually matter.
I simply can't sympathize with someone who is disappointed with writers who are unable to perfectly resolve the literally unresolvable "correctly". What, exactly, did you expect? Within the context of the show and the logic the writers chose to represent, the finale was fucking fantastic.
Besides, it's not like we've seen everything there is to see about what Peter did at this point. There's obviously a whole lot more to tell.