Glad to hear you are making head way on this one! Sorry I didn't respond sooner but have been too busy at work to come in here and help. The “only silly thing” left could very well be the relay for the compressor. Sounds like you found which relay is the compressors, code J in the fuse box according to my USA car wiring info. Little education on Bosch relay control, 30=B+, 87=Consumer, 86=control for relay (turn on/off) 85=ground. Now that you have power from the pressure switch we can focus on the relay control. You have power (B+) on pin 86 so that is good. We need to test 85 with the relay plugged we are going to perform a voltage test on pin 85. Black lead of your meter to battery negative (B-) red lead to pin 85. If you have B+ on 85 then you have an open ground, supply a new ground since that is where that wire goes. IF no then we need to make sure we have power on pin 86 with it plugged in. Move your black lead to B+ and red lead to pin 86, should show near zero volts or no more then .5 volts at the very most. If you have more then this you have a bad connection somewhere on the power side of the relay, move your red probe back through the circuit until you get that near zero reading, that is you point of problem. If all this fails to turn up any thing it has to be the relay. OHM test terminals 86 to 85 should get around 45ohms.
Hope this helps and make sense, outside of this I am out of ideas, sorry.