You are going to need an impact to get the clutch basket off. Do not stick anything through the fingers on the clutch basket unless you plan on buying a new one!
I originally had a smaller hole, but couldn't get a wrench and socket in at the same time. Now I have plenty of room, and the oil level is below the hole so when you open it up it doesn't pour out.
It sounds like you have the same type of locknut. I put mine together and had zero problems until I took it apart, and now that it's back together it keeps coming loose.
I like the 20" better for all around riding because it's not as tall. The 22" tires balloon up and make it harder to fling the rear end around without bicycling.
32:1 is plenty of oil, I wouldn't run your Banshee any richer than that. I have been using that mixture for almost 16 years now in my Banshee and no oil related problems at all, and my Banshee isn't ridden easy.