And finally - perhaps you might consider a more customised approach for the fuel bowser. Specifically the ability to set a 'refueling' flag, rather than have GSX attempt to control the FSX/P3D default fuel system (that we currently control through our own system logic).
I'll reply to your other questions later on (we might need to discuss it internally first) but, for the time being, I'll just reply to this one: could be you haven't noticed the option in the airplane customization window that tells GSX to bypass the default refueling page ? It's there precisely to allow for customized refueling procedures.
Note that, even if you don't set that flag, GSX still won't control the refueling in any case, it will just *display* the default fuel page, but it will then up to the user to change anything.