Additional information: So, when I replaced the defrost thermostat, just wanted to say that the evap fairly well frosted, but what was notable was that the bottom of the evap coils sitting in the defrost drain pan was completely blocked up with solid ice. I'd say at least 1-2 inches of solid ice.
Not sure if this helps, but it confuses me as you'd think this would happen ONLY if the defrost heater WAS working and somehow it's just building up at the bottom. Please note I did clean the drain tube, (to some extent). I snaked it with a smaller diameter vacuum line I had laying around down to about 10-12" inches.. Not sure that's enough..
Any helpful comments welcome.