A subspace shock wave emanating from a star, especially one that was apparently large and massive enough to cross interstellar space relatively quickly and destroy the Romulus system, yeah I can believe that. But Praxis? Yes, I understand what you're talking about, I suppose you could say maybe the dilithium in the moon helped cause that, but on the one hand, if something like that could really occur, I wouldn't want to be anywhere near it! Can you imagine the kind of detonation necessary to cause it? Whew!! Also, it seems to me that the damage to the Qo'noS system would have been far more extensive than just decimating the planet's ozone layer. One would think there should have been huge asteroid and meteoroid bombardment and probably ground quakes and all sorts of natural cataclysms that the movie doesn't mention. Not to mention the beginnings of formation of at least a temporary ring system.Now, obviously what the 2009 movie alleged to have happened to Romulus is not exactly the same as that, but that's where Praxis comes in. There we did see something described explicitly as a "subspace shock wave," clearly travelling faster than light and depicted as having harmful or destructive effects. That's not exactly the same either, but it and GEN do demonstrate that it's at least potentially possible for the effects of a supernova or cosmic explosion to propagate faster than light. So while the Romulus situation of 2387 is distinct from those events in a number of ways, those events do provide precedent for that general class of events, i.e. cosmic explosions that have some effects propagating faster than light through subspace.
Cast No Shadow does establish the existence of a Praxis ring in the 2290s.
As for the rest, well, I guess the survival of Qo'Nos as a heavily-populated class-M world after one of its moons explodes with sufficient force to propel material light-years away via subspace is Star Trek's equivalent of Star Wars' Endor Holocaust. (As (this technical page puts it, "What happens when you detonate a spherical metal honeycomb over five hundred miles wide just above the atmosphere of a habitable world?") Qo'Nos and Endor survived because they had to, for plot reasons.