I’m still processing everything, but for those who are wondering about the ending: Remember the video Peter found in the lab with his name on it in the second half of the finale? Walter recorded that shortly before (I’ll explain why it was shortly before) the invasion in the original timeline (the timeline we were watching before Walter and Michael walked through the wormhole). So, when they walked through the wormhole and time reset, and we (the viewers) went back to the park in 2015, the tape already existed. Then, once the exact moment came when the invasion had begun in the original timeline, but didn’t happen now, a new timeline was effectively created, and Walter was “deleted” to prevent a paradox (as Walter explained to Peter after they watched the video).
When Peter got home and was looking through the mail, he found the letter from Walter (whom he remembers; this isn’t like the Season 3 finale where Peter was completely erased from everyone’s memory; he might have spoken to Walter that morning), and opens it to find the tulip. Of course this is where the episode/season/series ends, but now remember the video. Imagine (as Walter did in the video) Peter calling to ask about the letter, then going to the lab when Walter doesn’t answer, finding the video, and watching it. I wish/hope Walter left more than that paltry video to explain why he had disappeared forever.
I’m guessing from the fact that Walter says in the video that he had sent the letter that he knew when the invasion/his deletion was going to happen, because it would have to be only a few days before Peter received it. Remember the end of the Season 4 finale when September appeared to Walter and warned him about the coming invasion? He must have known when it would happen. This is when they devised the plan, and September fragmented it in Walter’s mind. This is why they needed to get the Thought Unifier in Season 5 Episode 1.
There’s certainly more to say about the finale, but I’m spent for now. Comment with any questions or if you’re interested in a more detailed explanation. I might do one later.