Update: Last night, I could not find the missing SNG files by using BBM, looking on my computer and the pedal. But when I did a deep search via File Explorer today I found most of the missing songs, in a TMP directory. Interestingly, when I imported these back into a new project using BBM, I got early versions of the songs with names that reflected their original sources. (Many of these were based on Groove Monkee songs.) This was far better than creating them from scratch, so I feel quite lucky.
I also found some special characters in the song titles, leading me to believe that IS what caused the problem. I took all the special characters out, renamed the songs, and made a few final edits. Call this a success! I’m back virtually where I was when the problem occurred.
I don’t see a definition of special characters anywhere, so that’s one question for anyone who knows. But I’m playing it safe for now and taking out anything that isn’t a letter or number. A surprising number of songs have non-letter/number characters, so this isn’t trivial. Especially with material that I’ve purchased, like the Groove Monkee stuff.
Thanks for your help, and if I can add any more detail to clarify something for future reference (or code updates) please let me know.