Before Star Trek: Discovery was around, back in 2009, J.J. Abrams set up several mysteries during the first Kelvin Timeline movie. The imaginatively named Star Trek. The most significant one of these for the 'Prime' Star Trek timeline was the Romulan Supernova. But is Star Trek: Discovery Season 5 e...
Even the Romulan-heavy Star Trek: Picard Season 1 avoided addressing or explaining the supernova in any detail.
Explaining? Stars go supernova occasionally, there's your explanation. There doesn't need to be further cause or intervention behind it.
IMO this is a solution (or really several) looking for a problem that just doesn't exist. As it stands and despite fan speculation, Romulus was destroyed by a natural disaster, which is actually something that happens on Earth and in the wider universe.
Not spontaneously, with only a handful of years' notice, they don't.
I'm all for Trek science being a little weird, but it seems very likely that there was some funny business going on for the star to suddenly blow up and threaten the entire galaxy.
The wiggle room within Star Trek is that the Tkon Empire was supposedly wiped out by a supernova despite being a highly advanced civilization, and in "Second Sight" the DS9 crew boosts the top speed of a ship to warp 9.5, which suggests that a supernova can have superliminal consequences.
But there's nothing to suggest the Tkon supernova was a natural phenomenon, and the hijinks they were up to in "Second Sight" certainly wasn't.
Anything involving Romulus’ star means that ‘saving the planet’ is completely impossible.
Eventually, yes, but if it were possible to collapse the exploding star in a way that didn't totally roast the planet it would buy a fair bit of time to continue the evacuation. It'll get very cold and photosynthesis will stop, but with enough power and food, the population could hang on for several years if needed.