Uhh... it depends on the nature of the failure. You're assuming that the fuel would detonate all at once. It's equally likely that once being boosted up to that altitude, the third stage engine(s) lit off and immediately gimbaled hard over and locked there. There's a video of one of our Trident missile test launches where that exact thing occurred - but right after it cleared the surface of the ocean. Instead of roman candle, think "pinwheel".
I’ve seen those pinwheel failures, and they nearly always ended in an explosion. This Norwegian thing looked like something out of one of Tim Burton’s movies.