as for the question of damage to defiant . a theory about connie shields is that they were like layers of an onion. multi hits would take out the outer layers while still protecting the ship. but they could feedback energy in waves .. where all the shaking of the ship came from. sometimes even with shields a shot might get through a weak point and cause an actual breach. but you could have internal damage in the ship w/o actual breach.
Well, truth to be told is fundamental flaw in Star Trek. Ships always look damaged, but in the next scenes damage is gone. Star Trek VI had such a gaffe too, Enterprise having a torpedo passing through the whole saucer only to show no damage in the next scene.
Excelsior was actually the reverse. A mild hit that made no damage on the exterior, but the scenes show extensive decompression in the interior.