One subtle discontinuity seems to escape everyone about "In a Mirror, Darkly".
The I.S.S. Enterprise seen in "Mirror, Mirror" seemed to be very similar to the U.S.S. Enteprise of 2267. The controls and protocols we associate with the U.S.S. Enterprise of 2267 seem to apply perfectly to the I.S.S. Enterprise (with few exceptions, such as the transporter effect and the "agony booth"), right down to the sound and behavior of the controls.
But in "The Tholian Web" the U.S.S. Defiant seems to be a "currently updated" Constitution-class vessel for late 2268/early 2269, right down to the sound and behavior of the controls. (At least from what we can observe, if memory serves.) Therefore, the U.S.S. Defiant of 2268/9/"The Tholian Web" could not go back in time and become the I.S.S. Enterprise of 2267; it would have to revert specifically to 2267 specs. How could the Terran Empire do that so perfectly?
I would also point out that, while "In a Mirror, Darkly" was a fun romp, the Defiant as seen in that episode could not have been the same ship from "The Tholian Web". The Defiant from 2268/9 that Kirk discovered was a derilict. It was pretty obvious that she wasn't going anywhere on her own power, let alone fight off a Tholian squadron. If the ship could've been revived as quickly as Tucker did, Kirk boarding party would've reactivated the ship and flown her out of the disputed Tholian frontier. You remember what McCoy said: "Jim, this ship is dissolving! My hand just passed through a man and a table!"
I will say this for "In a Mirror, Darkly": Manny Cotto saw to it that the Defiant was awfully close to the one lost in "The Tholian Web". Great job on the bridge set, even if the controls on the bridge sound a little off. I do not have a problem with a fully operational Constitution-class starship singlehandedly cleaning everyone's clock in the Mirror Universe. By Archer's 22nd century standards, Defiant would be a giant! He would regard that ship as having a "Warp 8 engine" (or was it Warp 9?) and technologies on board that would make it seem like a Borg cube next to Warp 5 ships. If you accept the notion of the Defiant of "In a Mirror, Darkly" being the same one as in "The Tholian Web", then Hoshi has all the cards. Her super-starship could dance circles around 22nd century Warp 5 ships. It's over four times faster. And lest we forget: Defiant is equipped with replicators.
Having said all that, it was fun to watch, but the Defiant in "In a Mirror, Darkly" could not be the same ship. Not a chance. The whole story was a romp for me. I could not take it seriously. What a shame. It was half-baked like the rest of STAR TREK: ENTERPRISE.
And I agree to some extent - I liked IAMD, but it did open some worm cans that could have been avoided. ENT as a series did quite a bit of that. 
