you can always try to submit your game. nowadays games get accepted way easier (i mean they fucking accepted multiple different chess games after elo took over) but yeah visual novels will have a hard time being accepted due to the runs mostly consisting of you holding the skip button to go through the (normally) hours long text and the actual gameplay is just really simple
i used to use padpyght. works for all kinds of controllers and is highly customizable
i assume youve already tried it but does the game run on windows 10? i run/moderate couple late 90s early 2000s pc games and at least they run fine on w10
ive always mostly watched others play games (videos/stream) than play my self. it has always annoyed me when someone says "so you play video games" when i answer "im on my pc" to "what do you do on your free time" because of course, you cant do anything else than play games on a computer and most modern pc games suck ass anyway since they are basically all the same shooter with different art styles and maps. of course there are other games than shooters but no one talks about those for some reason.
nice rambling
usually i think you can rebind all you want as long as you dont give yourself extra buttons to do inputs (example two shoot buttons) but you should always do what bacon said and ask the mods of said board
not really what you asked for but: you could also try to do 2p1c (2 players 1 controller) runs. dont think that many boards have it as a category but that doesnt mean you cant do it:D and mods may add it as a category if you run it
this is the site's forums not portal's forums. those are located at https://www.speedrun.com/portal_2/forum
you should ask that in their discord server or forums.
ive wished for this for so long.(:D) and :D dont contain the same emotion despite being typed in the same way
or as it seems you are new to speedrunning in general. i hate to say it but i advice to learn some other category than 1/0 star its not a good starting point