I implemented your fancy knowledge number 3 (two musics in one event) successfully in FireRed US BPRE. However, I use Pokemon Ruby as my base, so I've been trying to adapt it to Ruby. I've found all the offsets to change and found all the pointers except for one. I could not find the one that's like "func_setvar"
Here is what I have changed so far:
This is what the main ASM function looks like after I changed all of the pointers: