If I were to make only one suggestion that would be to have a service tech look at the unit.
You have a few things that are probably a cascade effect of the origonal problem.
The origonal problem was most likely a defrost timer gone bad. When you defrosted the unit it solved the problem temporarily each time. The last time it probably ruined the fan motor, and that is why it isnt cooling as it should. If the air can not move then the only thing that happens is the condenser coil ices up, and in essence you have an ice box instead of a freezer.
As these repairs require quite a bit of disassembly I recommend that a professional do the repairs. It is not a novice DIYers repair.
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