When I play for longish periods of time, my thumb ends up hurting because of the sharp corners of the d-pad. Am I allowed to file just the corners a little bit, or would that be considered hardware modification and therefore cheating?

do whatever you want with your controller as long as it doesn't give you an advantage in game (like turbo or L+R)

I would probably consider that hardware modification. You're making it easier to make inputs which are therefore cheating. Honestly I wouldn't even change things because that just makes all of these rumors about people cheating and not getting real wr's

Well, I play with a PS4 controller and my runs didn't get rejected. It's not cheating at all.

That has to be allowed. If you want to get very technical, RGB modification is 'technically' hardware modification, but it is obviously not banned. I don't see anything wrong with it, especially since it's not allowing illegal inputs to be performed. A moderator's opinion though is always final so maybe my logic is very flawed but I don't consider this to be an issue.

You can play with literally anything, as long as it does the exact same thing as a NES controller meaning no left+right, and no turbo You may file your corners

It's not cheating. As long as you are doing the button presses, and they can only be done in a way that an original NES controller can naturally be done, it's fine (so no L+R, no U+D).

I have no issue with it either. It's a comfort level adjustment, not an adjustment that would give you an unfair advantage.