But Star Trek has never been 'Hard' science fiction of that type. It's always been more of a "Space Fantasy" with some grounding in actual science IF the story being told at the moment isn't affected.
As I said before, in its day,
Star Trek was closer to "hard" SF than anything else on TV. Everything else was pure fantasy and often painfully science-illiterate.
Star Trek actually consulted with real scientists and engineers, and though they often chose to ignore the scientists' advice in favor of a more fanciful approach, it was always an informed choice rather than the lazy ignorance of its contemporaries and successors. I doubt you can find anything in 1960s, 1970s, or 1980s SFTV that's anywhere near as credible as
Star Trek was at the time. Well, maybe the 1988 series
Probe, which was co-created by Isaac Asimov, but that lasted all of 8 episodes.
Part of the reason
Star Trek was so compelling to so many people when other SFTV shows fell by the wayside is that it was the only SF future on TV that felt even remotely close to plausible. So when people today dismiss it as being just as fanciful as its contemporaries, or even worse, they're misunderstanding why it was so important and special for its time. The only reason today's shows have surpassed it is because they built on the foundations it laid. In its day, nothing else even came close to its level of believability, even with all the liberties it took.
It just didn't make sense to me. I'm no astrophysicist, but the idea of another star in the galaxy going supernova and being an immediate threat to destroy a nearby starsystem is ridiculous, which is how it is portrayed.
Generations' whole plot depended on the supernovae triggered by Soran having an instantaneous gravitational effect on the Nexus from parsecs away. The Stellar Cartography scene showed the gravitational shifts acting instantly on starships parsecs away. Similarly, the
Excelsior in TUC was struck by the shock wave from Praxis right after it happened, even though there's no way a Starfleet vessel on routine patrol would've been just a few light-minutes from the Klingon homeworld. So as silly as it is, it's a canonical fact that cosmic explosions' effects can propagate FTL.
The idea that the supernova could threaten all life in the galaxy is also crazy.
A
hypernova is theoretically powerful enough that its radiation could devastate worlds for many thousands of light-years around, although it's now known that most of the effect would be concentrated into two emission cones along the magnetic poles.
Although I like the explanation someone offered before that it was the political chaos from the destabilization of the Romulan Empire that could endanger the known galaxy.
One area of interest, that you touched on, is types of star. For it to be a full, powerful, type II supernova, the star's initial mass would have to be nine times that of the sun, or higher, and the resulting star would have been pumping out staggering levels of radiation and energy for its entire life. Such a star would destroy itself before any planets could properly form, let alone develop into life-supporting worlds. It either wasn't a supernova in the conventional sense, or it wasn't Romulus' star.
Again, there's plenty of Trek precedent for inhabited planets' stars going supernova. It happened twice in TOS's third season alone, to Minara in "The Empath" and Sarpeidon in "All Our Yesterdays."
The plan probably was to use the Red Matter to stabilize the star, but time ran out so Spock hoped to at least use it to dissipate the wave.
That makes more sense than what was shown in the movie. I think we have to treat the images in the mind meld sequence as figurative, perhaps how Kirk's mind was interpreting Spock's mental impressions. (There's no way he could've seen Vulcan's destruction with his naked eye from the surface of another planet, so I've always assumed that was figurative.) The most sensible explanation is that they knew in advance the supernova
would happen, Spock tried to get to it before it happened in order to neutralize its effects, but it went off before he got there.
The Countdown comic has been categorically retconned out of canon by Remberance
It was never
in canon to begin with. None of the IDW comics have ever been claimed as canon, except once when an overzealous interviewer browbeat Roberto Orci into conceding they were canon just to shut the interviewer up about it, and then Orci retracted it in the article's comments hours later.
Countdown was never entirely consistent with the movie anyway, since it made Spock and Nero good friends when the movie dialogue seemed to indicate they'd never met before. It also made some bizarre, fanservicey changes to the movie's intent, like taking the Vulcan-built
Jellyfish and saying Geordi La Forge designed it.