Hi gulliver, thanks for your work!
It's the first time I try BetterWB, as I usually use 3.9 on my real Amigas, but since I don't have them here I decided to use a lean 3.1 install for WinUAE.
There's a problem though, when you try to change screen mode in ScreenMode, you get an ENV: error. I presume it might have something to do with the fact that you have implemented EnvHandler? Actually that's just the first thing off my mind, I tried changing some other Prefs and I got the same error. If you want I can take a screenshot.
I don't think I might've done something wrong on my part as I've just clean-installed 3.1 and then applied BetterWB 3.6 on it