i think mainly because of lack of interest in the mod as a speedrun. If a few people want to start doing runs and submitting im sure mods wouldn't have a problem creating a leader board for it

Mostly in loads as far as I'm aware, plus the lack of laggy rooms. I believe the JP version has most if not all JP exclusive glitches patched so it's essentially like the NTSC-U version on emulator with Japanese text.

The way things work with category extensions is we have a category extension extensions (lol) excel spreadsheet and if there are 5 submitted runs or more for a category it can be submitted for mod review to be added to the extensions official leader board. If you like i can help you get that added to the spreadsheet for you if you have a run to submit.

No, nintendont is not allowed as it sets the read speed to wii mode which reduces lag and saves a significant chunk of time.

There are other apps you can use such as gecko, Swiss, and priiloader which all can play games in gamecube mode and not reduce lag.