The fact that you aren't switching center vent position bothers me. The common heater problem is the mono valve but it won't affect the decision that closes the center vent. if its making that decision wrong it likely will do others.
One aspect of your symptoms that is OK though is the compressor function. it comes on only by evep temp in the normal position (unless the outside temp is less than the cycling temp - say 40deg F). In other words A/C is functioning to dehumidfy the air even in with the setting at "85".
Bosch Master, ASE Master, L1
33 years MB technician