1) Instrument types are already described or the sappy sound engine information doccument
2) The latest release of GBAMusRiper supports Golden Sun instruments, although one of them* (the one that is the melody of the battle themes) is not accurately reproduced. However your way to reproduce it is not accurate either. It's a square wave whose duty cycle permanently changes. How exactly it changes is complicated and mysterious, and independent of the pitch, so reproducing that in .sf2 is impossible.
The closest I could get is to have a sample of constantly changing duty cycle square wave, but it depends on the pitch, i.e. the duty cycle changes slower for low notes, and faster for high notes. In the real soundtrack it does not acts like this. Perhaps there is ways to make it "sound close enough" using completely different methods.
*) There is 3 types of Golden Sun instruments, 2 of them are simple (saw wave and triangle wave) and are accurately simulated with .sf2 format. I should mention this on the next update of my document.