Flaps? Where are these flaps on a 210? It's not a vacuum problem. Note that the OP is well over a year old. Perhaps his car is fixed.

There is one refrigerant circuit, and a one-piece evaporator in there, so yes, "1 main line for the" refrigerant. The temperature differential is achieved by running the heater core on the warm side, if required.

There is something to the low refrigerant causing a warm left side vent on the 210. Since I can't see inside the parts with the system running, I can only guess as to why this would be so. My guess is that the evaporation is taking place largely or entirely in the right side of the evaporator core, because the left side is full of oil. If it were full of gas, there would be some cooling on the left side, but often there really isn't any.

Another potential problem, of course, is the left side of the duovalve. This should cause problems on the left and right side with equal frequency, however, yet the warm left-side vent on the 210 is extremely common. You almost never hear of a warm right-side vent on this platform. Yet another potential problem which is left-right symmetric is the left-side heater core temperature sensor.