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Need to run a capacity test on the compressor. This is done by pinching the suction line closed ( there is a tool to perform this ) and see if the compressor will pull down to at least a 20 inch vacuum in two minutes or less. This being done with it's normal charge of refrigerant in the system. And then turning it off and see if it holds this vacuum. If it passes this test the compressor is functioning properly. Seems like the technician was on this track , but not sure what you meant when you stated " discovered the thicker bit in the copper circuit was blocked "
You need to examine the evaporator in the fresh food side of the fridge ( unlike most refrigerators this one has TWO evaporators , one in the freezer and one in the fresh food side ) This evaporator is behind a cover on the back wall. If it is covered in white frost - that's the problem and usually the problem is the evaporator sensor is defective and not allowing this evaporator to go into defrost mode )