5) How does 1 photon torpedo Destroy General Chang's BoP in ST6
As was said before, Chang's BOP took 5 or 6. It only took 1 to kill him. I think the single torpedo kill of a BOP you're thinking of is in Generations.
Someone said maybe only one or 2 detonated. But if these use matter/antimatter, which I'm not sure if they ever said they are photon torpedoes, then they should have all gone off if they were armed. If memory serves, the cryo tubes replaced the fuel pod. I don't remember if the warhead was missing to. But even if it was only one, it was inside the unshielded engineering hull where there is a lot of antimatter. That should have destroyed the Vengeance.