GDEMOSS: very interesting symptoms and diagnosis. keep me updated on your continued diagnosis and fix.

I got my AC working again through DEF mode. It turns out that there were 2 issues that stopped it from working:
1. The AC coolant (freon or 134) was low and finally tripped the pressure sensor.
2. Before I took it to a shop to test and add freon I was looking around and notice one of the 2 connectors on the regulator being unplugged so I plugged it in. Nothing changed so I forgot about it.

I woke up thinking about this plug yesterday and went out and unplugged it and the AC began working thru the DEF mode again. Apparently the original owner (who passed away) must of been troubleshooting this issue and learned that this would bypass the issue allowing the AC to work in DEF mode while he was continueing to troubleshoot the Auto Climate Control System.

On the Auto Climate Control System, the original (Deceased owner) must of changed a number of parts since in the boxes of stuff I got with the car were a number of used AC component parts in original boxes.

Now that I have AC again in DEF mode the pressure is off me now and I can get to a number of non-mercedes projects and trouble shoot the CLimate control system later when I get time.

If you need any information let me know. I'm not an expert (only beginning to learn about mercedes since my purchase of car 2 weeks ago) but a full factory service manual set came with the car including the "Heating, Air Conditioning Auto Climate Control" service manual.