The story as written does not hold together at all: the travel time references are massively self-contradictory. Thankfully, the movie execution is vague enough that this does not necessarily result in an internally impossible fictional chain of events.
the supernova destruction is not a problem if we assume it to originate near Romulus.
We really have to assume it to originate at
Romulus to meet two simultaneous conditions: Spock stops the supernova at a location where Nero meets him (that is, right when he sets for home in a fast ship, so it can't be a location different from where the supernova was stopped), and Nero witnesses the destruction of Romulus firsthand, at visual range. If Spock stopped the supernova at the putative Hobus, Nero would not be there to intercept him.
...And never mind how Spock could reach a location appreciably deeper into the spherically expanding wave of supernova destruction than Romulus is, considering the wave rips planets apart and Spock has but a fragile little ship.
The one out from this is to say that an explosion at Hobus can be neutralized by creating a black hole at a distant Romulus. Which is very difficult to accept or explain, especially considering your next point and the symmetry concerns inherent in it.
Comparable Federation losses, which we might expect if the Hobus location were moved to a border, are conspicuously never mentioned.
...Indeed, the closer we move things, the more understandable this becomes. If Spock stops the supernova "at the bud", within an extremely confined area of space, not only does it become more plausible that this is accomplished by a single black hole, but it follows that the destruction never reaches any
of the rest of the galaxy regardless of the symmetry concerns of the situation.
Spock would then have to cover a shorter distance
Indeed. And since distances can always be fought with high speed, the only good reason Spock could be late is if he is fighting reaction
time issues and travel
time is not a concern. Shorter distance is a more plausible explanation for failure to reach Romulus in time: if a long distance were involved, Spock would be able to calculate things to perfection and would be there in time (or would realize he's going to fail and would not go to Romulus at all), yet if he is
late as seen, he must have been surprised by the events, which is likelier to happen during or before
a short, desperate trip.
An explosion at a distance, countered by a long trip that takes too long, results in a host of problems. Chiefly, Spock looks like an idiot for traveling to Romulus for nothing, because stopping the explosion with a black hole away from the core of the explosion would now be better done in the direction of some other vulnerable system.
A local explosion, countered by a short hop from Vulcan (but too late, because the explosion happened sooner than Spock thought - remember, most folks didn't even believe
in an explosion, highlighting the uncertainties involved), would wholly remove the problem and be fully consistent with the idea that Romulus lies close to Vulcan. (Also, it would allow the fastest ship of whomever Spock considers to be "us" not to be all that fast after all, by general standards - making it more plausible that Nero would give chase to it in his own lumbering juggernaut. Although we can easily attribute that to Nero being crazy as a cuckoo at that point, too.)
Given the timeframe of the film Nero has plenty of time to get to Vulcan sometime the next day.
But this is a contradiction of internal logic. Starfleet took a certain if unestablished time (from San Francisco morning to less than the next San Francisco 22 hours, apparently) to get from Earth to Vulcan. Yet Nero fails to outrun our heroes in the opposite direction, despite Spock giving him lead time by making the detour to Delta Vega. Getting from Spock Prime's point of emergence to Vulcan between the timestorm of 22 hours and the "soon thereafter" when Starfleet got a distress call from Vulcan is not gonna happen if Spock Prime's point of emergence is farther from Vulcan than Earth is. So we either have to accept Klingon space as being closer to Vulcan than Earth is, or Klingons being in Romulan or Federation space. (Unless we wish to discount the rumors of Klingons being massacred altogether - Nero would be a likely culprit for launching such rumors, as he must engineer Starfleet's departure from Earth and Vulcan for Laurentius somehow. But that's just another possible apology we could create to remove the inherent contradictions of the movie, rather than something we need to build on.)
The Narada does get from Vulcan to Earth before our heroes; we see that in the film.
starts lowering the drill into Earth while our heroes are still underway. But Nero hasn't activated the drill when our heroes beam aboard from their ambush positions in Titan's clouds. So the best we can say in Nero's favor is that Nero reached Sol at the same time with our heroes, who were limited to warp four and gave the villain lead time by making the detour.
(Actually, the editing must be confusing the issue; the arrival of our heroes at Titan must be concurrent with Nero's drill-lowering antics, not coming after it. Otherwise the time gap between the drill being lowered and being activated would make no sense. But that's just icing on the cake.)
(Really, this is probably an editing error
: our heroes originally spoke of ambushing Nero as he passes Saturn
, so showing him at Earth before our heroes are at ambush positions runs contrary to the dialogue. Plus it establishes our heroes using extremely long range transporters, which Kirk already had bad experiences about. But it's probably more difficult to negate an outright editing error than to point out that two things shown happening subsequently are just as likely to happen concurrently. Although of course there is no overall need to argue that scenes in a movie would take place in the order they are shown.)