Lee Son of Pete wrote:
Apologies if I'm missing something, or this has been done to death (this is my first day on this board) but I think all we know for certain is that Nero, his crew, and Spok believe that Romulus is destroyed.
You're forgetting the chronology of events. First the supernova went off; then Spock undertook the red matter mission to try to stop it; but he was too late, and the "shock wave" hit Romulus before he could get there. Nero witnessed the destruction of the planet firsthand: "I watched it happen! I saw it happen! Don't tell me it didn't happen!" Spock then used the red matter to somehow "stop" the supernova at the source (even though that should've done nothing if the radiation had already spread far enough to affect Romulus), and in Spock's own words: "As I began my return trip, I was intercepted.... In my attempt to escape, both of us were pulled into the black hole."
Spock was apparently aware of the planet's destruction, since the next thing he said after relating it was "I had little time" to deploy the red matter, suggesting that he was hastening in reaction to the event. So we know that Nero (and presumably his crew) directly witnessed the destruction and that Spock was either notified of it by someone else or detected it on long-range sensors. That's enough corroboration to be confident that the event actually did occur as described.