Spock would not know if Universe B's alternate time-line was created before or after Nero's actions.
Nero is the alternate factor and him just using the so-called time-ship may have and most likely have created the alternate time-line which is Universe B. Nero's actions would be apart of the Natural history of Universe B because it is an alternate time-line of Nero's actions and Spock's actions to stop Nero would not be the Natural history of Universe B. Spock is the one braking the Natural history of Universe B, not Nero. Spock is also braking the Temporal Prime Directive and the Temporal Agents of Universe A would come down on him, not on Nero.
In The City on the Edge of Forever, Captain Kirk acts to preserve the timeline by allowing Edith Keeler to be killed in a traffic accident. Spock is the one who keeps telling kirk that Edith Keeler must die. Spock is the one character in Star Trek who really follows the Temporal Prime Directive. Like I said Nero is the alternate factor and therefore must continue he actions just as Edith Keeler had to die. If Nero (the alternate factor) is left to do as he pleases and shift Universe B into a kind of a nightmare universe, then it is up to Universe B to stop him, not the Elder Spock of Universe A.
Now, if you read the Elder Spock with Scotty scene of this Abrams trek movie you can see that the writers have dumb down spock and has him breaking the Temporal Prime Directive left and right in this film.
What Bob Orci wrote as his alternate time-line theory for this movie is just dumb. Bob Orci's alternate time-line theory is made just to be confusing to people who do not know physics theories or even Trek physics theories.
This is how JJ Abrams works, He makes his dumb stories and his bad writing confusing to people, so people think "I don't know what is going on... so, it must be smart". You can see these actions of JJ Abrams in play on LOST. JJ Abrams and his writers are just Dumb-asses who are confusing people to make them look smart.
Hell, even MadTV as pointed this out about JJ Abrams and his his writers:
http://www.youtube.com/watch?v=ixC0HTQgGB0&feature=related