Looking back at the thread, your last post of Nov 30 said you would try to clear out the air passages and if that didn't work, then a total defrost. I also suggested you try to run the diagnostic cycle to see if anything was not functioning. Did you defrost the unit and run diagnostics and if so what did you find out since last Nov? The freezer way below zero would indicate the compressor just keeps running as it thinks it is warmer than it really is, but the much lower temperatures in the bottom of the freezer still indicates trapped air. You said you had airflow through the diffuser which would indicate it is working, but you may want to revisit this area. Use a mirror and see if the fins are open or closed. If you open the freezer door you will see light through the diffuser. The diffuser connector to the motor may have failed, or the motor itself may have failed causing the cold air to pool in the bottom of the freezer. My guess is the diffuser is working because the normal symptom would be a warmer fresh food section as the thermostat that controls the compressor is located in the air duct from the freezer. One other thing that comes to mind, however, is you mentioned you had service on the unit. I have seen conditions where someone removed the diffuser and in the process, dislodged the temperature thermistor so it was actually sticking into the fresh food section resulting in the compressor running all the time. It's probably time to step through each component one at a time until you find the failure. Make sure all air passages are clear and then check the diffuser as suggested above.
This may not be your exact tech sheet for your model, but it is for your model family. Do the tests and see if the thermisters test OK. If you have done the defrosting and that didn't make any difference, then you are down to the thermisters and control so hopefully the diagnostics will point in a direction.