I was thinking you need to find the defrost sensor. It probably is in the form of some sort of thermister. The board needs to know when to defrost and yours isn't defrosting and you have replaced the terminator which shuts down the heater and the heater and the board but so far you have not replaced the defrost sensor have you?
Here's what you can conclude from your tests:1. The heater is OK2. The timer is likely not the issue, since you manually initiated a defrost with no heating.3. I would replace both defrost thermostats.The temperature thermister has nothing to do with this problem, so don't test it for anything.
As Icehouse said, let it run a few hours. The thermostats you replaced have to be cold to send power to the heater, if everything else is working OK, the heater will come on.If you put your AMP meter on a single incoming power wire at the rear of the fridge you should read from 3-5 amps when you initiate defrost.Good luck.