Jeesh... They just used a little artistic license for the sake of storytelling. Haven't any of you seen one of those movies where the dead person narrates what happened? It's a little cheezy, but a small price to pay to get Leonard Nimoy's voice in the game.
I don't think Spock is dead though. I think at some point he will make an appearance in the game. I could be wrong though. Once he corrected the time line he might have tried to return to his own time and overshot and that's why we are getting the overview.:p
The mirror universe is an parallel dimension, not an alternate timeline. There is a distinct difference. Otherwise in Mirror, Mirror, Kirk, McCoy, Scotty and Uhura wouldn't have changed places with their counterparts because it would have been a different time. Kirk even says that it is a parallel universe. They never mention it being an alternate timeline.
You say that like it's a scientific term with a precise meaning. Neither turns out to be the case. However, if you want to try that route; how come all the folks in the JJverse end up in the same roles, despite histories diverging so much that they've got different relative ages?
Answer: because these universes are linked in some way. Just like the Mirror Universe.
My theory is it becomes such a "strong" quantum signature because of the observers from the other timeline remaining in it. Thus, most alternate timelines don't reach a state where they can travel to the Prime Universe, because the damage is undone and the observers leave them. The Mirror Universe, however, must have had those who caused its formation remain in it, strengthening its ties to the Prime Universe. The same, therefore, would hold for the JJverse, since Spock remains in it instead of "fixing it" and thereby leaving.
This actually holds some distant, not-really-science-but-science-ish relation to some real cosmological theories, regarding the formation of universes from the pre-big-bang "quantum foam" that came and went due to no observers ever coming into being within them, with ones like ours becoming full-fledged universes due to the fact we someday were to exist in them.
If memory serves, the gamma rays from this sunburst was big enough to wipe out any potential life in its galaxy, and the four surrounding galaxies. It produced a measurable surge of gamma rays here on earth, in our own galaxy. Heck, it might have been the inspiration for Abrams' supernova.
Fortunately there are no such stars in our galaxy, or in the galaxies around us. Neat, huh?
Spock narrates the events that follow AFTER he goes back through time and establishes the alternative timeline. How can he know these things happen? It doesn't make sense.
We can't say that alternative timeline Spock returns to the future either, because he would be going to the future of the alternative timeline, as established by Back to the Future (well except for the Biff part).
I can't wrap my brain around how Spock narrates events that he cannot know about.