I have done a search but cannot find a thread on how to stop the slewing
You can't, and the problem is not GSX going into slew mode (it's done for a reason, of course, to be sure the airplane height were you edit the doors with it's always the same and not depending on your actual weight), but your JOYSTICK, which is not calibrated properly.
Either adjust the joystick null zone, or turn it off before editing the airplane (using CTRL+K)
Additionally, how does one designate the catering truck to pull up at the opposite service door on a PMDG 747-400F which does not have a service door Fwd R?
You can't, GSX supports only catering from the right side.