Guys, the number one rule of Star Trek fandom is, don't try to make sense of time travel physics.
It's pretty clear from the context of the movie that in the Abramsverse, nothing exists from the original timeline except for Spock.
Ok, ok. There's one way Spock can get back to his original timeline. Go back in time to 2015, find Biff Tanen and eat his brain. Then he will steal his superpower to go back in time, change the past, then get back into your original unchanged timeline.