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The damper must be bad and not opening to let cold air flow into the fresh section as I assume there is no defrost issue and no ice blocking the return air paths. If you look at it you should see the door on it as open IF it is working; if it's closed then it's bad.
I have made you an offer for the call. I will be around most of the day today but I don't sit here staring at the screen waiting for replies either, so be patient.
The coil is totally encased with ice- this should not be. You still have no defrost taking place, so you still have a failed part that makes this defrost.
You must have a bad connection somewhere as the heater needs to get power when in defrost..... IF all your visible and accessible connections are fine then you have a non-repairable problem here as you cannot fix wiring buried within the cabinet. I assume here that the defrost thermostat is cold enough to be closed when you tested this.
Get a voltmeter and see if you are getting power to the heater or not when you cycle this into defrost. The heater should have continuity. I pretty much assumed that you knew that the heater was fine as it would be highly unlikely it's bad snce you have replaced it previously. Replacing parts with no diagnosis is not going to be productive, so get a meter and make the check.
I am looking for a rating at this point.
I would appreciate a rating for the help I gave you here. You have not asked anythimng else, so I can only assume I helped you get the answers you needed.