the ice was built up in the lower section with the heaviest in the lower right corner.
With this statement I would have to say you have a refrigeration problem if all the defrost system checked ok. Considering you do have frost on one section I would say low on freon but it could be a bad compressor so I would check the compressor before throwing it out. Make sure the condensor fan is running and it doesn't appear the fan is slipping on the shaft or anything. If you have an amprobe you can check the running amps to see how much it is pulling also. Listen to the compressor and make sure it is not starting and stopping often also as this will cause one to heat up. I found a write-up for you on how to test the pins on the compressor to see if the compressor test ok.
The total resistance between Run and Start should equal the sum of Start to Common and Run to Common.
Example: C to S = 5 Ohms, C to R = 2 Ohms, Then C to S = 7 Ohms
Testing for Grounds: Using an Ohmmeter place one probe on a good ground
such as the ground screw on the compressor. The other probe should then be placed on one of the pins as so stated. If you get a reading, the compressor is grounded.
Testing for open windings: Using an Ohmmeter take the leads and test between any two pins. Reading infinity compressor has open windings.
Testing for shorts: Using an Ohmmeter, place the leads between any two pins. Reading zero shorted compressor.