When the misc categories say that NG+ files are allowed, does that just mean that you can have already beaten the levels and just have to show beating them again? Or is there some way to reset your save data without resetting your cars that I'm unaware of?

1. Yes. NG+ profile in this case means, that you can have already beaten levels and just beat them again. This game has a weird progression, hence NG+ profile.
2. I have no idea if there is a way to do that. If you find anything like this, please tell us.