Any simple by-pass will leave you without control. That system can really make some heat if you put it full flow. Being in Atlanta I don't think you will like it.
I remember years ago seeing in A/C catalogs manual heater valves which looked just like the bottom of the MB/Chrysler servo. These I presumed were for Chryslers that didn't have Climate control. Seems like a lot of trouble though. How much real diagnostics have you done on the system. Does the system switch duct positions?
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician