Good to hear that you're making progress. My project Cosmo has the combined steering / oil sensor issue - which fuse was going on yours (the big one seems to be intact in mine, so I assume it is the one in the fuse box that powers the steering controller and a few other things that may have tripped)? The air-con valve connector is a new one on me despite all of the reading I've done so if you've got any advice on whereabouts this is and how to get to it (any dismantling needed etc.?) it would be appreciated - does this impact on aircon function at all?. Bear in mind I've also got what I am certain is a leaking diesel pump on the top of the engine so my issues could also be secondary to diesel running over wires below- I have the Bosch replacement and gasket but it is too dreich to get out and perform the swap at the moment.
I had the same symptoms as yourself - I changed the power steering pump but that didn't help. I just disconnected the electrical connector on the air con valve and that solved my issue. Try just disconnecting it first and see how you go.