He's also in another Universe and is a different character entirely. He's no more Prime Spock than Intendent Kira is Prime Kira.
I cannot see NuSpock as being from another Universe. In that way the movie doesn't work for me. I think this movie takes place in the same universe we have always loved and that the time line has been permanently altered.
Go ahead and believe that, but you're wrong. The writers fell all over themselves trying to tell us that it's a new universe, going to the lengths of inserting clunky dialogue into the characters' mouths. I really don't know what more they could have done, other than to stop the movie cold and walk on-screen to directly tell us what they had in mind.
And why can't you see Quinto and Nimoy's Spocks as being different characters? They're played by different actors. We've seen cases where the same actor plays different characters (MU, etc) so it's not like there's no precedent.
That chunky dialog was ambivalent at best. For the entire movie NuKirk and NuSpock et al. acted like Spock Prime and Nero were from
THEIR future and not some parallel universe.
For me it is no big emotional payoff to have Spock Prime witness the destruction of Vulcan if his planet Vulcan is safe and secure in another universe...Nero gave the impression he was going to have Prime Spock suffer by watching his planet Vulcan destroyed...but if it wasn't
his Vulcan that was destroyed that scene looses all emotional impact.
In the mind-meld scene Spock Prime tells NuKirk he is from 129 years in the future, he doesn't say he is from another universe or a parallel universe.
"City on the Edge or Forever," "Yesterday's Enterprise", "First Contact", "Past Tense", "The Visitor", "Time and Again", "Future's End", "Before and After", "Year of Hell", "Timeless", "Relativity", and "Endgame" all established that tampering with the past DOES change the present and does NOT create a parallel universe.
My main point is this: Whether the movie is in a parallel universe, alternate reality, alternate timeline and what all of that means is still being debated. I think the writers, who did intend to establish this movie as being in a parallel universe, were as clear as mud about this...which is why there is still debate about it.
I can and do see both sides of the argument and do understand why people choose which side of the argument they are on because both side do make valid points.
But in the end the debate is pointless because in the future the writers can do anything they want from clearly establishing that the old Prime Universe does indeed still exist, to resurrecting the old Prime Universe if it doesn't continue to exist!
But for me the movie works much better on an emotional level if I view it as a movie that just creates a new time line that erases over the original one.