Thanks for all the very helpful input.
I had no conflicting copies. I restored an older version of the db (through Dropbox) that showed a lot more content (per its size), and, weirdly, it lost all of its tags again
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I ended up restoring a version that was an hour or so older. Once the db was restored, I checked it with Caliber, then re-imported by hand every folder that showed in the folder list but not in the book list, It was quite painful. Then I reentered the missing tags by hand - altogether an afternoon of work, but a lot less than I had invested before, thanks to the forum feedback, which made me realize that all the metadata is in the single db file.
I can't help wondering what caused the tag corruption. Looking at what tags got corrupted, I cannot figure out any way that I could have somehow screwed them up through the graphical interface, which is what I was using. I am worried that this might happen again...
Clearly I am going to be pretty anal about hand backups from here on. I could not find a simple way to set up calibre for automated backups of the some kind - did I miss something in the manual? It's very easy to do it by hand, though.