4 months later. Mapping 2 buttons to one control is more to prevent for A+Y (item dashing). SNES9x also has a designation for the diagonal mapping. The only thing you need to do is make sure that "Allow Left+Right/Up+Down" is unticked.

the real way to softlock 9 is to drink a potion while entering the stairs.

I probably won't play the game much until the next tournament

Gnarly has 8 dragon coins, there might be another level that has more than 5, but that is the first that comes to mind. Then there is a single dragon coin in Chocolate Secret [CS].

When 5 dragon coins are collected in a level, you receive a 1up, but also the dragon coins will no longer appear in the level. So the level with 8, nothing tracked changes after the 5th coin (you will receive a 1up for each additional coin over 5 you pick up), and in CS the coin will respawn if you enter the level again.

Anything 1.5x or newer since 1.4x and earlier is banned. I think the current iteration is 1.53 but I don't stay up to date on that emulator.

I recommend Higan.

the hardest part of SMW on emulator is boxjump in YI3, this has to do with the input lag ldad mentioned. Once you get comfortable with that, you'll generally be used to the input lag.