I am working on a theme for GMB to work on PinePhone Mobian. So far it's working pretty well, but double-clicking doesn't work well on a touchscreen device. Mobian will probably create a universal fix, at some point, but for now, is there a way to set it so the track activates on single-click, not double?

The problem with that is you need to be able to select the row without activating it.
Are you using the gtk3 version (still in beta) ? It might handle touchscreen better.
Also are you using the SongList or the SongTree, the SongList use the standard TreeView gtk widget so it might work better (though it is a bit slow with lots of songs in the gtk3 version).