I've checked, both are true. There is one bank per tileset, and the MIDIs are linked to them in a SLK. I was incorrectly remembering that you can make a MIDI-based sound in WC3 using any file path, you can't, what you use instead of a path is a sound label which corresponds to an entry in the MIDISounds.slk table. That table then stores the directory name, track name and bank name for each label.
The question is, can we overwrite this slk by importing a custom one? That way, we could make more than two tracks share the same bank. Alternatively, we could put all the tracks into a single file and then play the sound at different offsets to get the random shuffle feature.
I think a far bigger challenge than getting it to work in WC3 would be to produce the track and bank files in the first place. If you wanted to use a minimal number of instruments, you'd need to write the tracks yourself, anything you can get off the internet just wouldn't work since different tracks can use different instruments, so even if you find tracks that individually don't use many instruments in the end you can still end up with a huge bank if you want to support all of them.
But who knows, maybe iNfraNe would like the challenge of writing some SW inspired music. ;)