If "hobus" is Romulus's own primary, Spock's plan doesn't help and would probably screw the planet just as thoroughly as the supernova. If "hobus" is not Romulus's own primary, there should have been months at least from the time the star goes nova to the time Romulus is in any danger. In either case, a supernova is a process that takes millions of years. The Romulans would have had a lot of warning.
There's no getting around it. This part of '09 has enormous plot holes.
its not an ordinary supernova, obviously. it took out a planet. on screen. it was said to have threatened the galaxy. its just a supernova with fantasy elements. like genesis or praxis and the multiple other strange phenomena in star trek