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.
I agree that it was a serious gaffe, but maybe the explosions were actually sparks when it comes to hull of an advanced ship.
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.
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.
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.