Yes, the defrost thermostat looks OK.
What did the defrost heater read?
If both OK then odds are that it is the adaptive defrost board.
You said the evaporator coils were iced up in one area, this does not point to a defrost problem.
For defrost to cause problems the coils have to be frosted/iced up to the point that the evaporator fan cannot pull air through them to cool it properly.
Frost in only one area points towards a low freon condition.
Here is a link
Check out #20 and the 3 example pictures part way down the page.
If this is the case it will be an expensive repair as you will need a pro.
Often it makes more sense to put those dollars towards a new unit.