I agree that it was a serious gaffe, but maybe the explosions were actually sparks when it comes to hull of an advanced ship.One serious gaffe in IAMD part 2 was when the Defiant had sustained direct hits which apparently violated her hull when she battled the Avenger; yet after the battle there is no visible damage and it seems like the battle never happened. That was just plain ridiculous.
On this one I think you are wrong. That made sense, a lot of sense. Tholians detonated a tri-cobalt torpedo in the gravity well of a neutronic star and it opened a rift between dimensions. They had no idea it was also a time rift. They learned that only when Defiant was lured in. Tholians wanted to lure a ship from another dimension to learn if it is safe to go through. The fact that it was from the future and that passing through the rift drove the crew insane was just a nice bonus.I absolutely do not buy the whole Tholian scheme to lure a ship from a future they shouldn't know about subplot. That was goofier than TNG's Sela story arc.
And actually Mirror Universe concept was introduced in Star Trek by Roddenberry. After all, Mirror, Mirror was his creation and in that episode the people were even crueler than in IAMD. Advancement in career was always made by killing people above you.
I sorta wondered if in non mirror verse the tholians almost the part of self appointed time police while their mirror side where trying to make use of any temporal tech they could find.
the tholians in the non mirror verse were more concerned with warning enterprise away. even then they allowed them to stay awhile to see if kirk would reappear. they didn't get pissed until the deadline was ended. even then their web seemed for containment and not destruction.
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.