The sensible thing to do would have been to use the red matter to destroy the sun there and then (assuming there was no other way and he didn't do it in the 25 year gap) before going after Vulcan and Earth... Nero wasn't sensible though, he was just out for revenge.
On the other hand, he did yell at Pike that he prevented genocide.
Since he did this some time after gaining access to red matter, he might well have saved Romulus before launching on his revenge spree. Assuming, that is, that Romulus could be saved with red matter, although the specifics of that still evade us.
Assuming the whole Hobus thing was unavoidable then Romulas is probably toast in whatever timeline.
It probably wasn't "obviously unavoidable", but more like climate change, so that all sorts of "supernova-critical" voices could dismiss the threat till the last second. Both the 2009 movie and PIC imply such denialism, after all. Or at least dismissal of a specific hero effort, even though people who truly believe they are going to fry should be very happy with a belt-and-suspenders approach.
But there's no "Hobus thing" in either the 2009 movie or the PIC episodes: the latter simply says that "the Romulan star" blew, supposedly establishing that it was the homesun of Romulus. Applying red matter to that a century before the kaboom might not be healthy. And if red matter were available to assorted villains, then any planet that cannot shoot down a lumbering mining rig whose drill is sensitive to handguns is at risk of disappearing into a black hole. Including, quite possibly, Vulcan again.
Original series and the Next Generation did "The Naked Time" and "The Naked Now".
And we have TOS "Wink of an Eye" and VOY "Blink of an Eye", the latter changed at the last minute. Plus, of course, TNG "The Emissary" and DS9 "Emissary", and TAS "The Eye of the Beholder" and TNG "Eye of the Beholder".
Timo Saloniemi