This unit has a standard defrost timer that can be turned clockwise to initiate defrost.
Some of these models I believe have the old mechanical timer and some have the defrost control board.
What I would have done is to ohm out the heater and the defrost thermostat before defrosting everything. You know the heater works by the way you tested it.
The defrost thermostat should test 'closed' or shorted with a meter when it was covered in frost/ice if it was working.
If both the defrost heater and defrost thermostat tested ok with a meter I would go ahead and replace the defrost board.