...Plus, they knew the special beaming technique worked on two people (more or less). Recalibrating something they had no real understanding of might have been a costly mistake!
The supernova can't be a normal event or there would be years to fix it. Of course, if was ftl then Spock would have to match its speed and stay ahead of it (he appears to be flying towards it). How Nero managed to get there at the exact same time has to be placed in the same box as how Kirk stumbled into Spock in a cave. The universe did it.
If the supernova happened ten lightminutes away from Romulus (i.e. it was the Romulan homestar), it would only be natural that quite a few Romulan ships would be around the place at the crucial hour, and that some of them would be saved by Spock's stopping of the expanding destruction.
In that case, the event could obey known laws of physics, which is always a bonus if it doesn't endanger drama. Furthermore, Spock would be aware of those laws (such as lightspeed propagation of destruction), so him being late would be the result of him guessing wrong on the detonation moment. If the window for rescue were mere minutes, then a fast ship would also be useful because travel time to the rescue location would be significantly longer than the rescue window; Spock could plausibly place a few calls, secure a preexisting supership, and take off and head for Romulus when (but not before) the threatening burps from the star made the impending calamity too obvious to be ignored even by stubborn Romulan and Vulcan politicians.
Also, the supernova could be stopped by intuitively appealing "symmetric" means, by dropping the red matter at the very center of the phenomenon while still being in close proximity of planet Romulus. Proper timing would allow Eisn to die a dignified death that made possible the evacuation of Romulus; a black hole would be a less immediate threat than a supernova wavefront, and it's even possible that Eisn might be stabilized rather than blackholed if Spock were able to perform his stellar surgery with due care.
The single major problem with that interpretation is that the supernova explosion would hardly threaten the entire galaxy physically. Yet dialogue makes it clear that the explosion did present a danger beyond Romulus, because Spock still had to act after Romulus was lost.
The two-step dance around that would involve saying that the galaxy was threatened by the political ramifications, not by the wavefront; and that Spock had to act to save the rest of the Romulan star system (perhaps including Remus, which seems to orbit a bit farther out as seen in ST:NEM) or because the blob of red matter he had extracted and primed was unstable and would blow on his face unless he dumped it somewhere (and dumping it into the supernova would be logical, as it would stop further calamities in the decades ahead).
Timo Saloniemi