i see no on screen evidence of a satisfactory conclusion to this arguement
That's why we should consider what the writers intended. Obviously they want the TOS characters to play with, but young and hot instead of old and wrinkly. They also don't want to be concerned with crap like knowing that Kirk can't die for a few decades, that Scotty gets frozen till the 24th C, that McCoy lives to be 120, or that there's a Doomsday Machine on the way to hoover up planets.
So to free themselves up, they whomp up a new universe that is similar enough to the old one that they can call it Star Trek
, but where the future is unknown and therefore fun. The only question is whether this universe existed before the incursions, or came into being because of the incursions, but that's a fine point that has no bearing on the fun part, and therefore is unimportant.
Works for me. More to the point: it works to resurrect Star Trek
, and nothing else would have been guaranteed to work remotely as well. It's good that we got Star Trek
back this way because we wouldn't have gotten it back any other way.
Would the Spock who went back in time in COTEOF to prevent McCoy from changing the past, and who insisted that Capt. Christopher had to be returned in TIY to preserve the future; be so willing to write off the destruction of Vulcan and all the inhabitants, when he knew that this was not as history should proceed?
You're mixing up time travel within one universe with travel between universes (with time travel happening there too, just to make it all the more confusing.)
Kirk & the gang time travelled within their own universe in COTEOF
to save the Federation because it was their Federation, and they'd grown to like it, plus they needed a ride home. In Trek XI
, Spock's Vulcan was perfectly safe back in his own universe. The Vulcan that got blown up was a different one, native to this new universe. The existence of two universes (and really, it's at least three - have to count the Mirror Universe) may imply the existence of an infinite number of universes, in which case there are infinite numbers of Vulcans, some of them getting blown up and others not.
Once you realize that, fussing over one Vulcan seems like an incredible waste of energy. Infinite universe theory holds that everything that can happen, does happen, in at least one universe. Vulcan can be blown up; therefore it's destined to be blown up, somewhere. Maybe this universe is that place. Trying to stop it from being blown up here guarantees it gets blown up somewhere else, so the whole thing is an exercise in futility.
Or, maybe Spock decides, this universe is my home now. Let's see if I can undo the destruction of Vulcan, since regardless of whose Vulcan that was, people died and that's bad. Whether that causes them to die in some other universe is unknowable so I won't worry about that. So after the movie ends, Spock tries to time travel within this new universe. Maybe that's his project for the rest of his life. Maybe he succeeds and maybe he doesn't. Maybe he does time travel but can't stop Nero. He can try again and again until he dies. All that is beyond the scope of this particular movie.