Yours is precisely why I do not work on Samsung nor LG refrigerators. Both companies over-complicate an already complicated system. You've already replaced the parts i would check, the defrost heater element, the therm which mounts at the top of the evaporator, though the temp sensor is not part of the defrost system, I still would have tested it, and if all those passed, I would test the system continuity via the main board, and if that passed, then id replace the board.

However, for clarity, is this snow like frost evenly thick throughout the evaporator, or is there ice buildup somewhere on the evaporator.

that main board contains the defrost control. I checked my sources, and i found a service manual for you. you can find it at the url below. hope this helps

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try going to appliance guru dot com