Thanks for the tips!
So I took apart the horn, replaced the diaphragm with an old one from that Chorus pair (I did those up with Ti diaphragms the week after I got them home back in January), and it sounded even worse! But I think it was due to the fact that tweeter diaphragm had been loosely stored in the Random Junk drawer in the kitchen for months, banging into all sorts of stuff. I'm sure that voice coil is probably dirty as well as damaged. Plus, that magnet air gap looked dirty.
Theoretically, if I clean up that magnet, possibly install ferrofluid (should I?), and deck it out with some fresh Ti diaphragms, it should absolutely work 100%, correct? The wiring looked to be absolutely good, the crossover looked fine, and the rest of the speaker components look perfectly good (except for the veneer -- that is another project for another day!). There isn't any way it's a "bad" magnet, right? Pretty sure it's not possible but I'm not an expert on such matters so figured I'd ask here!