The assumption of STXI is that the "time travel" depicted in the film involves a branching universe, so from the POV of the film "transport Spock in time" and "transport Spock between universes" are the same thing.
I don't think that assumption is warranted. My understanding is that "branching" is an ongoing process that has nothing in particular to do with time travel. The results of time travel just provide one more possible cause of that branching. From the point of view of the black hole for example, (if it were able to!) it neither "knows" nor "cares" whether the target universe will divide or not.
Thus I can't see why time travel shouldn't operate in a very traditional fashion. Its only when something gets to the other end of the wormhole and "upsets" the reality it arrives in, that a new universe is produced.
It's possible that as Nero went through it created something like a "wake" in FC terms - a connection to the new timeline which also affected Spock's entry.
In FC the "wake" is just a by-product of the temporal field the Borg ship generates and the Enterprise is "protected" by being within it. Other than that, there is no connection between the two vessels. Moreover the Enterprise goes to the same time and place as the Borg. By contrast in STXI, the plot requirements of having Spock appear 25 years later makes the behaviour of the black hole seem more random and Spock and Nero more unconnected. In any event, the black hole is already performing the function of the "wake" or field: To deliver Nero and later Spock to the past of the Prime Universe. I just feel you are asking too much to expect it to somehow override that function and change Spock's destination in response to events that occur once Nero has passed through.
The creation of a new universe seems like it would be more likely to server any connection between Spock and Nero that may exist. I would not expect the Enterprise to appear in the same universe as the Borg ship, for example, if that situation had been based on a branching assumption. I would have seen it as a technical error, assuming I noticed it! Still, if you are happy to see the STXI version in the terms you discribe that's OK with me.
However I feel the most likely explanation is that the writers weren't aware of that problem and just got it wrong. If you are looking for an in movie fix, the
pre-existing universe theory is a much better fit in my opinion. By the sound of it we have heard the last of red matter or at least time travel for a bit anyway. Personally I love time travel's ability to set a scene, but relying on it as a critical device in the plot is problematic. Or has been without branching realities.
UFO said:
I think it helps if such plot devices are at least semi-plausible.
That's exactly the problem I have with "traditional" time travel.
I agree with you. I have no problem with the branching "solution" as a plot device generally (though I am not sure of its real world implications). I just think it was the wrong choice in light of what we see on screen. By the way, if you want to travel in time and return home in a
branching multiverse, you would have to have the technology to do the actual time travel and to hop between realities. Even so, going back would split your original universe in two as well. Not a problem really as that is happening all the time anyway.
I think it's the best way to do it and it extends the lifetime of the franchise dramatically, if in another 15 years someone wants to come along and present their own take on Star Trek they can do it.
Absolutely!
If we assume it was necessary, then it was a better solution than a complete reboot in my opinion as there is still some feeling of continuity.
The many-worlds theory (at least the one I've heard of) doesn't hold that time travel creates anything, but that infinite parallel realities exist because that is the nature of the cosmos. So time travel does nothing, and in fact, time travel doesn't really exist.
Actually
constant branching is another of the many-worlds theories. Time travel may or may not exist, but as mentioned above I think it is a separate issue. By the way, in a non-branching, no time travel multiverse, I believe that in order to show the events in STXI you would have to "assemble" samples from a number of different realities at different stages of development. I.e. Nero couldn't be the same one Spock Prime follows through the black hole.
The way things like time travel and multiple universes may or may not work in real life need have no bearing on how they function in a work of fiction.
That's true, but I think good science fiction is an extension of what we know about reality rather than a contradiction of it, unless there is a very good explanation.

I'm not sure "red matter" qualifies as an explanation? Then there is that other exotic material that (according to countdown I think) somehow converts a supernova into a galaxy destroyer.
Star Trek is fantasy, and has been since day one. It's idea of multiverse and branching timelines can be however the writers want it to be at the time.
You could be right, but I prefer to think it is sometimes what passes for good SF in the video media!
*****
Oops, I believe I stuffed the following up.
OK, technically I believe it's a scientific axiom that you can't ever disprove something ...
I think that axiom is that you can't prove something, you can only keep trying and failing to disprove it!