Shouldn't mystakin's 100% run also be at the top of the Any% All Music Sheets leaderboard, since he collected all the music sheets during it?

I suppose, but first he'd have to submit it

You can submit it on his behalf. Also, I think the category should just be called All Music Sheets. Any% can have restrictions after it, but All Music Sheets is a requirement rather than a restriction. You see things like "All Missions" or "Any% No Major Skips" or "70 Star" referred to as "Any% No BLJ". Any% with a requirement after just makes the Any% bit of it irrelevant.