Well... As said... Not much to troubleshoot...
Everything works as desired, from A/C operation to vent control. No codes set. I entered the HVAC diagnostic mode right after turning on the ignition, right after starting the engine, immediately after the LED started blinking. The error code layer of the menue constantly reports "00" - no codes present. This is really strange. Maybe an error with the HVAC control unit diagnostic mode itself?
When all refrigerant was removed the A/C compressor didn't engage as you said - BUT there was no Code 9. When I refilled the system it immediately started working after a certain refrigerant charge was reached. No battery disconnect needed.
I think I'll observe it for a while to see if it narrows down to a point. Really strange...