Not sure but the PP shows that sometime the plate get stuck, fig 4 & 5 in the link
Pelican Technical Article - Mercedes W210 - Heater Control Valve (Duo-Valve) Disassembly and Cleaning
I see you are selling the old duovalve, would you say that after cleaning it, it is now a viable fix for the issue you were having. I ask because my HVAC is flukey-Hot HOT on side vents with AC on and only cold air coming out of the center vents and sometimes not so cool air on the drivers side. I have recharged the AC, but still have this issue and thinking cleaning or buying a duovalve is the next step.
Thanks.