Thanks for the fast response, but it didn't work unfortunately. I think I'm going to try making note of where all my files are in my modded copy and just doing a hard format.
If I go through one by one adding things and checking Mewtwo I'm sure I can figure out what's causing the crashes, it's just how [censored]ing painful it is to be combing through everything at all.

I'm also having some issues with my Homebrew launch Project M and I've been fiddling with it on and off for actual months to no avail, so at the end of my rope I've come here to beg for help.
So I changed 3 sounds in the joucyu folder, and almost all of Meta Knight's sounds and nothing else. Everything runs fine, with one exception, when Mewtwo gets picked, the game crashes either before the stage loads, or as soon as the camera pans over to him, presumably because the sound effect of his "round start" animation crashes the game.
I was confident that installing the memory leak fix would resolve things, but to my chagrin it hasn't. Can anyone help me understand how I can fix things?
For reference, Meta Knight does not crash the game in any capacity, no matter who Mewtwo's opponent is, the game crashes, and Meta Knight's sawnd file is well below the cap of what's allowed.