As for emotional impact, knowing Amanda might still exist in the multiverse didn't lessen the gutpunch I got when she died in STXI. From this POV, she'd dead.
I was wondering, did you realise Amanda was a "copy" when you first saw STXI? Her death occurred before the "alternate reality" clue was discussed, so perhaps you thought she was the "original"?
The other issue is whether it would mean as much to someone in the movie who knew it wasn't "their" Amanda or Vulcan. It would still be a shock I should imagine though Spock Prime didn't see her die of course.
Spock clearly remembers a different past for James Kirk that James Kirk remembers, which tells me that events cannot unfold as Spock remembers them, in and of itself a paradox, and thus not possible.
That is the classic time travel paradox. Usually we ignore it or assume it is possible in some way we don’t understand. What? Time slows down the faster you travel? Ridiculous!

Granted that’s not a paradox, but there is a chain of causation that produced the people that go back in time. It just gets destroyed "after" its produced them (from one point of view)!
Causality excludes a linear timeline as a physical possibility.
Hmmm, maybe. Anyway, nice reasoning.
I didn't see any evidence that any of the characters could know what happened - straight-up time travel, changed timeline creating new universe, or new universe that was always there.
No, not much. I figured Nero would have to understand the physics of it in order to work out where/when Spock Prime was going to show up and if he did Spock Prime should too. I was assuming the late 24th Century knows more about parallel universes and how or if branching happens than the 23rd. But from what
KingDaniel wrote, perhaps not. Spock Prime should at least understand the side effects of red matter but perhaps he doesn’t know what happens when you get to the past?
All I know is that the writers don't intend us to believe the old universe was changed and put facts in the brains of the characters regardless of whether they could know those facts, for the purpose of reciting dialogue to communicate the writers' intentions to us.
I don’t see how you can be sure of that from what’s in the movie. I refer you to my post
#74. Much of the movie still seems to retain legacy lines from when the writers appeared to be going for a same universe story.
OneBuckFilm’s reasoning makes "branching/alternative pre-existing" seem likely. But such reasoning is traditionally ignored in ST. If someone at the accademy had briefly mentioned doing an assignment on parallel universe physics and branching, it would have helped (I know, and slowed down the movie to a crawl!

). I can see that keeping it unknown may sometimes work but the writers told us afterwards, so why make it mysterious in the movie? Perhaps they thought they had done all they could. I disagree.
New universe vs pre-existing is up for grabs, so I'm opting for pre-existing simply so we can have the Eugenics Wars take place in the 90s in the original continuity and fob all the "corrections" such as a 21st C Eugenics Wars (ie, our universe) onto the new reality. But it's definitely a new reality one way or the other.
I think pre-existing is the best solution for how SpockP and Nero can end up in the same universe as well (not that its a burning issue

).