Hi drkillabrew, your observations are absolutely correct and therein lies the design flaw of all (most) these top mount refrigerators with ice-maker on the top. In order to be able to make ice and keep it frozen in a compartment in the fresh food section, they have to add an extra evaporator in the fresh food compartment. Without adding an extra defrost heater element, they rely on passive defrosting to defrost this evaporator.
I think this defrost sensor detects if it is getting too cold for too long and then shuts off the fresh food compartment evaporator and fan. I don't know if it also shuts off the freezer evaporator and compressor and turns on the freezer defroster, but since it seems they have separate controls for all the components, I don't think it would. Then it relies on the fact that the refrigerator compartment is above freezing temperature and that would melt whatever ice got accumulated. If you've ever left ice in the refrigerator, you'll know that it eventually melts, but not that fast so this thing should keep doing it's job or else a big block of ice will never melt. When I started noticing this problem the first time, I just unplugged the fridge and let it melt over night. After I plugged it back in, everything started working normally, but I noticed this time that the fridge always seemed to stay on and never completely shutoff. It was probably doing that before when something failed the first time and I didn't pay attention to it. Of course, it started icing up again. I hope it is due to this defrost sensor or thermistor being bad and replacing the evaporator fan unit will solve the problem. If not, I'm thinking of somehow rigging up a defroster to this evaporator, but I guess it would be dangerous if it isn't timed with the evaporator actually turning off.
I wish these things were not so complicated with so many components that can fail. I mean what happened to keep it stupid simple? I guess consumers are to blame for wanting these bells and whistles that the appliance companies are throwing at us. Also, the appliance companies like all this complexity and design flaws since it keeps all the money flowing from all the replacement parts and keeps repair technicians busy and in business.