the therm usually works or doesn't
sorry if i ask instead of reading back but are you sure the fan by the compressor is turning and there is no buildup of lint etc on the condensor by that fan?
your frost patern looks "loose" which indicates a possible weak compressor, but there is no way i can instruct you to test for this, also all your door opening from repeated checks could cause this loose frost as well
the defrost therm is less than 20 dollars and takes minutes to change, and it comes with instructions, you might want to change it out just to be certain it is not the fault
boards are more likely to be intermittent than the therm, but the therm is alot cheaper so i suggest you start there
if you do change the therm.... dont open the fridge unless you are getting something to eat/drink
remember what i said earlier.... your fridge decides when to defrost based on door openings and run time, your constant checks are going to increase defrost times beyond the norm