Why is it in multiship battles like this, a single torpedo, a single phaser shot sometimes takes down a whole ship?
Why not? If all ships were equally powerful, there'd be no point in building different ship classes. But when you build a BoP, you are making a compromise: you get a great light offensive unit, but starbase-strength phasers will blow her up with one shot.
Really, that's only consistent with "Return to Grace" where ground-based ordnance again essentially takes out a BoP with a single shot.
Besides they don't really aim the torpedos. They just fire in a straight line, its more luck if a target gets in the way!
How can we tell? There'd hardly be any point in flying the things in curved paths if a straight line will result in impact...