You get centuries warning if your star is in a natural decline. The Romulans only settled that world 2000 years ago, so either they thought it was fine to colonise a nearly blown up planet, it was just temporary, they were out of feul, or what happened to the Romulan Star was completely unnatural.
Maybe the Romulans inflicted their singularity technology on their own star, and it went bad?
I was under the impression that it was the Hobus star that went supernova, not Romulus own star (although the supernova endangering/destroying Romulus is not impossible).
Supernovae in real life can cause massive devastation from lightyears away (they'd just take much longer to get there and affect said damage)... the Hobus one probably went FTL with subspace interaction (and as you say, the ROmulans themselves could have been responsible).
Furthermore, I was actually wondering... where are the Romulans in all of this?
They have their own fleets of ships at disposal, so I don't understand why were they relying on the Federation for help?
Sure, the Federation being the good guys DID devote a lot of resources into this, but when the Romulan saboteur commandeered the synths and attacked Mars (and destroyed those support ships), I doubt there would have been enough time to send anything... still, what also doesn't make sense is the fact the Federation would have large number of shipyards in every member species star system that joined the Federation.
The Feds would have the capacity to construct thousands of ships at the same time (if each member species star system has its own shipyard facilities similar to Utopia Planitia and the construction yards in Earth's orbit - and likely in orbits of other planetary bodies in SOL and other member speices planetary systems), and have them rendezvous with each other at Romulus.
Constructing 10 000 support ships would be a very small task when looking at the scales from this point of view... and automation can construct them much faster than manual labor can.
But again, I'm having to wonder... where the heck were the Romulans with their own fleets?