I think what we got here is two forms of time travel being present in Star Trek, the traditional version where you go back on your own timeline, and any changes made tend to have catastrophic results, and the lesser-used one where a change creates a parallel universe. Frankly, I think "Parallels" is being misread in this instance, since Worf really isn't doing much time traveling, just jumping from one timeline to another while still progressing forward at the same rate; he doesn't really do any time travelling until the end.
Yes, I think the classic Quantum Mechanics theory of "many universes," where alternate timelines pop up spontaneously every second, is what "Parallels" was depicting. That's a whole different issue from using time travel to intentionally create an alternate timeline.
I think a better example would be the opening scene of "In A Mirror, Darkly", where Cochrane blows away that Vulcan, thus creating the Mirror Universe.
I don't know if that's an example of a divergent universe -- I think maybe the Mirror Universe has always existed alongside the "real" one. (Did the backwards universe you see reflected in a mirror exist before the mirror was made?)
Cochrane shot the Vulcan because he was already Mirror-Cochrane, and Terrans have always been hostile in the Mirror Universe. (See the alternate opening title sequence of "In a Mirror, Darkly," which starts out with old pirate ships in centuries past, and continues with fighter jets and submarines, to Neil Armstrong planting the Terran Empire flag on the Moon, up to the I.S.S. Enterprise NX-01, implying that the Mirror Universe has existed for centuries, if not forever, long before Mirror-Cochrane shot the Vulcan.)
As for why Nimoy's there, I think I'll go so far to speculate that while Nero screwed up by shifting into an alternate timeline, the old Spock that shows up used the standard linear approach (after all, it's his timeline that's being screwed with; it's doubtful that our Spock would even be aware of Nero's little scheme).
Since we haven't seen the movie, and all we know is that there's an Old Spock, we have no way of knowing whether Old Spock and Nero even came from the same timeline.
Maybe Nero came from the "Star Trek: Nemesis" timeline, but Old Spock came from the alternate future that Nero created, to come back and undo the changes Nero made (like old Jake Sisko in DS9's "The Visitor," or Tasha Yar in TNG's "Yesterday's Enterprise"); or, like Picard in "First Contact," Old Spock was following Nero into the past, and thus was able to see the changes Nero would make but was somehow immune to them. You are right: Unless Spock and Nero went back to the past timeline together through the same time-warp phenomenon, then Spock would simply cease to exist in Nero's alternate timeline and would have no way of stopping him.
For that matter, who's to say that when Nero went back to attack the Kelvin, he arrived in the right timeline? That ship really doesn't fit all that well with the design asthetic of the period, so I think it can be argued that Nero's little escapade is just one cosmic fuck-up after another.
I think that falls under the filmmakers' "artistic license," just as the Enterprise in the previous 10 films was "spruced up" for each film, both inside and out, depending on the whims of the art director for each film. I don't think starship models, costumes, makeup, and even other actors in the roles can be used as evidence in continuity debates (or else Saavik from STII having blue eyes would be from an alternate reality from the brown-eyed Saavik of STIII.)