I want to run 500 Unique Coins (no main stages), but 1) I don't have a 120 Star file, 2) the only leaderboard-allowed Linux compatible N64 emulator (Mupen64Plus) doesn't allow Gameshark codes and I have NO idea how to actually complete 120 star, I've only beaten the game using BLJs, and 3) OBS lags my laptop, so the audio from the game is super slow/laggy/chopped up, and listening to the music like that is a nightmare, so I turn off the in-game music with Usamune when I livestream, and play copyright-free music in the background while I practice. Can I use the Usamune ROM to run this category? I will not use any of the special options that would not be there in the original game (aside from disabling the music), and will livestream the attempts.

No, you cannot use it. you could use the program called "stardisplay" by aglab2 to give yourself 120 stars and the b1/b2 keys, then just open all the doors and save the game so you can start running

for the OBS problem, i'm not really sure how to fix that. try lowering your bitrate maybe?