I don't see any problem with that one. A supernova that destroys Romulus is obviously a threat to the entire galaxy, even if it destroys nothing else. After all, Romulans are likely to be royally pissed off by that one, and you don't want to piss off the Romulans.
Spock spat a tiny little droplet into the already exploded star with the result of creating a black hole. That probably wasn't what he had planned to do originally, because he had much more red matter with him. We don't know what would have happened if Spock had dropped the whole load into a supernova that had not yet exploded. We don't know that it would have consumed the star; for all we know, it wouldn't even have created a black hole in those circumstances. And even if it did, having a star with a black hole at its core might not be such a bad thing after all.
Just think it through. If the star explodes, there will be death in a matter of minutes. If the star is consumed by a black hole, this is likely to take thousands of years, if not millions. And the star will certainly keep on shining till the bitter end; if one shut down all fusion in our Sun today, the light would only go out a million years from now, because that's how long it spends on its journey from the fusing core to the surface of the Sun.
The science is actually on our side on this one... Whatever properties this fictional red matter has, if seeding of black holes is one of them, then it really is likely to be a good supernova pacifier. Supernovas already have what amounts to a big attractor at their core: they collapse inward before they explode outward. But the outward explosion is essentially a back-bounce from that part of the material that won't fit inside the collapsing core. One may well prevent that back-bounce by eating the material faster, in a controlled manner.
As for the matter of evacuation, it's obvious the Romulans wouldn't evacuate until they really had to, until they had absolute proof. An approaching explosion wouldn't suffice. But an explosion that was turned into a slow and controlled collapse would; the Romulans would have a few centuries or at least a few years to evacuate, now that they really understood they absolutely had to.
As for Remus being missing, I actually hail that development. Data's PowerPoint demonstration in ST:NEM shows that the planets don't orbit each other: they are depicted as being on separate orbits that brush against each other, supposedly at regular intervals. It's probably a similar deal to that of Vulcan: Vulcan has no moon, but every now and then a neighboring planet and its moons come really close...
The supernova simply caught Romulus when Remus wasn't in the neighborhood. Nor was there a companion planet visible in "Unification". In turn, Shinzon in NEM obviously timed his slave rebellion so that the two planets would be in alignment.
Timo Saloniemi