I dont think they will even attempt to explain how Kirk lives.
I think the movie will start in the 24th centuary.
Spock has moved back to earth and now lives at Kirks old appartment in san fransisco.
He hasnt changed a thing about the way Kirk had it, and Spock does some kind of deep opening monolouge as the camera pans all the way around him to show his is alone in the room.
THe film starts in proper and he does all the wacky shenangians in the past with the new versions of the crew, then at the end of the movie he is back in the room again, back in the 24th centuary.
He starts another monologue "I did the best I could to maintain the timeline as it was, although logically just being there, and the romulans being there, there would be some subtle differences to the new timeline."
All the time the camera is circling the room with him in the center as it did before
"How ironic that when the romulans went back in time to stop the begining of Kirks life, they would actuall end up undoing the end of it"
And the camera finishes its circle and Kirk is stood next to him, obviously older than he was in generations, but clearly alive and well.
Kirk holds two glasses, he passes one to Spock.
"To Bones and Scotty" Kirk says warmly.
They clink glasses and the credits roll.
That way you get Shatner in the film, and he only needs to take about an hour off Boston legal to film it.
Plus the end leads it open for either a new film with the old cast, or the new one.