Yes, you're doing things wrong when matching headphones. To properly equalize 612 and hd600 for example you have to take into account that these have a completely different upper treble presentation with different placement of upper treble peaks and it can drastically impact this "resolution" and "soundstage" etc etc. You really have to do this the way OP did, using microphones and measurements to precisely match every single part of the FR or at least bring them as close as possible and even then there's no guarantee that you're gonna have a same FR for both at the eardrum.

Then i guess the damage is not significant enough to cause any audible changes, if there's no distortion - you're good.

Use a full sine sweep for testing and pay attention to any unusual sounds especially in the bass region. I think it's doable with the tape but you'd have to be extra careful to not apply too much force.

I have bad news for you...what you've dented is actually not the dust cover but the driver diaphragm, and that will most likely affect the sound. This can potentially be fixed with a little bit of suction applied to those dents once you remove the cover and gain access to the driver.

Looks like an ES series Audio Technica, i think it's an ES7.

I have both. The main difference is the corrected treble response on the CRA+ which no longer sounds grainy, harsh and overly accentuated on treble heavy music and songs with vocal sibilance present. As for the bass, i'm using short and wide bore Moondrop Spring tips on mine, and the bass response is generally similar aside from the fact that on some tracks the CRA+ has a touch more warmth and midbass presence. I don't think the bass ruins anything on both CRA and CRA+, but then again i find iems like ety extremely limp in terms of bass response so i can see people thinking that these are too bassy.