I know woofer dust covers do not emanate sound. I have a feeling that tweeter domes are the media that drives the sound. I hope I am wrong. If they do drive the sound, how bad are these?
In direct answer to your questions …
Yes, in tweeters the domes are equivalent to the cone of a larger driver. The tweeter voice coil is glued to the edge of the dome.
I've seen tweeters dented near the dome's center. They didn't sound noticeably different. But your tweeters are marred at the dome's edge. If that partially detached the dome from the voice coil, it could be a problem. I can't tell from a photo.
It's possible that the seller was genuinely sick. And it's possible that he's lying. My first guess is he sent a third tweeter because he knew he was sending visibly marred tweeters – that might be damaged. What price did you pay for those M&K tweeters?