My friends and I (during non-coronavirus periods) enjoy playing multiplayer trackmania together and I'm wondering if multiplayer recordings would count as permissable submission to the IL charts? We usually play in either split-screen or hotseat modes

I don't see why not since the basic timer and checkpoints work the same (but should ask a mod anyways). Wanted to mention that online multiplayer is available for this game, so your friends can play remotely with you as long as they have the game on the wii (not emulator, sadly). How do you do this when Nintendo Wi-Fi Connection (WFC) is shut down a long time ago? Simple, through Wiimmfi (https://wiimmfi.de/) it is simple to set up by following this video (read the description as well) , you don't even need to homebrew your wii! Keep in mind that you will have to perform some of these steps each time you turn on the wii. For me, and most likely someone who tries it will encounter a error when connecting to wifi multiplayer in the game, just make a new profile and that should fix it (your old profile did connect to the WFC at some time making it not work with Wiimmfi)

If you obtain an error code, and do not know what it means, look it up at: https://wiimmfi.de/error See who is currently playing the game: https://wiimmfi.de/game (not too many people play/realized there is online multiplayer for this game)

Yes, that would be allowed if you and your friends play on console. Both split-screen and hotseat should be ok. Make sure to either edit down the video to only include the submitted run or make some timestamps. Also, if you play split-screen, make sure to write in the descreption which player/screen the run is on.