I am no AC expert either, but I recall that there is a pressure sensor that cuts the compressor off if the refrigerant pressure gets to a certain point. If this is malfunctioning, it could cause the problems that you describe.
I had a 928 that you had to start with the AC on if you wanted it to work. You could not turn the AC on after the car was started. This turned out to be the pressure switch.
I sympathize with you. I just bought a 300E with dead AC.
Please advise us of progress.
Falls Church VA