I'm having random problems with my SDL AC compressor too. I still can't make out the pattern. I rigged up a simple relay to use while I test the various inputs to the Klima relay.
The only bypass I can think of is to replace the Klima relay with a simpler relay. The Klima relay compares the AC compressor speed (2 wires) with engine rpm (1 wire). You can't disable the engine rpm feed because the engine has to be above 600 or 630 rpm before the AC compressor will come on. You can't disable the AC compressor rpm feed if you don't disable the engine rpm feed. If you put the simplest relay that turns on the compressor when the PBU says to turn it on and use the wires at the Klima relay, you'll keep the minimum pressure safety switch in the loop. Besides the difference in rpm safety feature, you'll give up the turn off AC compressor at 115C safety switch and turn off AC compressor at WOT switch. You can gain back these features with a couple more relays. I don't think you'll disrupt the fan functions if you lose the Klima relay.
A counterpoint to the AC compressor speed sensor argument is how many other cars on the road with serpentine belts have this feature? To me it's like leather and ACC in an S-class - if it were my choice, I'd have velour and manual AC.