Jon Freeman wrote: The mobi winds up at over 200MB ...
...But Calibre can't view its own conversion...

My Calibre (v6.0) converted the 26MB epub to a mobi of 535MB. And does open it. Perhaps yours was incomplete? Other ordinary books I have are only slightly bigger in mobi format.

Jon Freeman wrote: ...epub is just a collection of html and svg files in an archive...

Unpacking it, I see that the the SVG files (4329 of them) are 77MB and the rest (9011) 47MB. I wonder if the mobi has converted the compressed SVGs to some other image format, each at several sizes for different Kindles?

The sqlite database of DT is 24MB. Poking about in it with sqlitebrowser I can see is contains lyrics in html (8982 records), tunes is ABC and SongWrite (4020), and keywords (711). (Mudcat's version appears to have 10365 songs.)

Your prototype webapp displays the lyrics in staff notation and can play midi - both from the ABC presumably. It could display, and offer to download, the ABC and the Songwrite source. Made into a proper webapp, loadable from the web but with the ability the run offline, it would be pretty neat.

I estimate that the posts in the Mudcat forum occupy about 5TB. That's not including other entities - threads, people, ... or indexes. Perhaps 10TB in all?