You have a bad damper control, or a stuck (frozen or obstructed) damper.

Couldn't tell from the drawings. The only visible board of any kind appears to be located in the rear, near the compressor, probably along the right side as you face the compartment. The damper itself will be between the freezer and refrigerator sections. If it's a motorized damper, you'll need further diagnostics to determine whether it's the motor or the board.

That could be exactly where the damper control is. It's a gamble at this point. If you can expose the damper motor and apply a test voltage to it, you may have a more informed choice. You'll need to access the schematic to see whether it works on high voltage, or low.

No wonder the drawings were confusing...I had only seen this in Sub Zero units. Let's start over: You say the refrigerator freezes. You'll need to remove the panel in the rear, inside the refrigerator, to expose its evaporator. Is it covered with snow-like frost?

If there are two separate systems (one for the freezer, and one for the refrigerator), there will be TWO compressors. If your unit only has one compressor, the evaporator for that unit is INSIDE the freezer, covered by a rear panel. If your unit has two compressors, the evaporator for the refrigerator is INSIDE the refrigerator, also covered by a rear panel. The evaporator is (usually) Aluminum. That's the coil I want you to look at. In normal operation, there is a uniform frost coating over the entire coil. In a faulty defrost situation, you will generally see a very heavy snow-like frost build-up all over the coil, and sometimes enough to make the rear panel bulge inward (toward the food).
CAPITALIZATION is for emphasis only...I'm not shouting.