I was never aware of D-Pad Up+L either and obviously it should be allowed. Great find! I might give this category another shot in the future. I made it up when I unlocked everything on my Japanese cartridge.

Sounds great! I'll go ahead and modify the boards this week.

EDIT: Done!

There is no leaderboard for beating all challenges as fast as possible. The category has been run previously by Boss:

I managed to find a save file with everything unlocked (including Ludwig, Huey, and New Orleans) using Project64:

https://web.tresorit.com/l#7V3hTNonrWh8rfjfkKZwoA

I recommend starting with one of the more accessible Ludwig courses: Los Angeles or New Mexico. Indianapolis is not too hard, but the tunnel part can be tricky for beginners. San Francisco is not too difficult and should be a comprehensive course to improve your sliding once you get a basic feel for it.