@cscat: Mind checking if this ft build
(Although I doubt it would fix anything besides being able to open the file, FT's is charge of the font rendering, not the layout. That falls to cairo/pango/harfbuzz. And that would be slightly more involved to switch without breaking everything
Also, your original message is still confusing, because it mentions mobi6 snapshots, while your latest attachment only includes mobi8 stuff.