Unless you got a frost free type freezer, anytime you are introducing fresh air into the system, your going to get frost, i.e. the humidity in the air. A totally closed system would work better and be a lot more efficient.
Right, a closed system would cause the compressor to run less often. You really need to use a thermostat to regulate when the circulation fan comes on or at the very least be able to regulate the fan speed manually. In any case, you def need a return line to equalize air pressure between the two boxes. Refridgeration and AC isn't about deliverying "cold" to places. It's about removing heat. Your return lines actually takes warm air out of the ferment box and into the main unit where the heat is removed.