You’re confusing yourself, and me. You’re either running mwm (the only version I know is the one from the lestiff project) or you’re running FVWM. FVWM doesn’t have a default “MWM compatibility” offering in the sense you’re describing, so you need to ascertain one thing, and one thing only:
Are you running MWM or FVWM?
The test for this is simple - and I think you’ve done so already, but:
ls -l $(which mwm)
ls -l $(which fvwm)
You can tell if you’re running inside FVWM, since by default you get a menu by clicking on the root window with a list of FvwmModules, etc. This won’t be FVWM.
It mentions FVWM because it probably allows you to switch to running that WM in place of MWM if it’s installed. At least that’s what the MWM menu allowed for last time I used lestiff’s MWM, a few years ago. I can’t think it’s changed much since.
So based on what you’re telling me, you’re running MWM and not FVWM. This is evident from the fact you can’t turn off edge panning.
When your next question is: “How can I make FVWM look and feel like MWM?” then let me know, since I like the MWM look myself and have written a fair amount about it.