15-16/"Mirrored": Retells the events of the '09 movie in the Mirror Universe, which doesn't make a lot of sense to me. I guess the idea is that in some alternate version of the Prime universe, Nero and Spock's use of red matter sent them back not to their own past, but to a parallel past. Yet that isn't adequately explained. I guess the handwave was that there's an infinite number of timelines in which all possible combinations of events happen, but I don't find that idea very satisfying. Anyway, since hardly any of the story is set in the regular Abramsverse timeline, there are no continuity issues with the sequel.
This is actually the correct application of Many Worlds and no, it isn't particularly satisfying. Essentially you are ALWAYS travelling back into a parallel past and that is what Spock Prime did in Trek09 as well. If it had been his own past, nothing would have changed going forward and the timeline would have played out just like it did in Time's Arrow.
You can't change the past because it's already happened but if you enter a parallel past, you become part of that past and whatever events you 'change' compared to the alternate timeline you came from would always have happened because you have always been part of that past in that timeline.
It may be unsatisfying but it sure as hell makes more sense than Voyager's time travel mechanics. Universes don't pop into existence when they branch. Pre-existing but overlapping timelines simply diverge.