The Fresh Food Section of my fridge is 50 deg F and the Freezer section is minus 20 deg F. What is wrong?

I have a Whirlpool fridge (model GS6SHEXNS00 built in 2005). I have checked the fridge through and through. The thermostat measures infinity at room temperature, the heater measures 20 ohms and does heat up. The front grill has been cleaned, the condenser fan works, the evaporator fan works. The evaporator coils are snowy and the drain hole is plugged with ice. I even placed a copper wire on the heater wire to prevent the drain hole from plugging up, to no avail. I have defrosted the coils manually with a hair drier several times. My last resort was to change the control PC board which has the defrost timer built-in. The new board did not fix my problem. I have noticed that the distribution vent between the freezer section and the FF section is not blowing any air from the freezer side. Therefore, no cold air is coming from the freezer side. That's probably why the freezer section is much colder than it should be (it should be about 0 deg F) and the FF section is warmer than it should be (it should be between 35-40 deg