Yes the unit uses these thermistors.
They measure the temperatures but since your temperatures are OK for some time after a manual defrost I would say that they are OK.
The unit does not have a separate ADC or defrost timer. The circuitry to control defrost is on the control board. That is why your problem is such a puzzle since you replaced the control board.
I do not know where you can get a service manual. I tried on the Frigidaire service website and they do not list one.