In my last apartment, the fridge was WAY old and had a spot on one side that showed up as frosty or wet, depending on the temperature in the house and the humidity level. I assumed it was where a bit of insulation was shot (believing, perhaps incorrectly, that the cooling liquid is only circulated at the back side of the fridge) and allowed more cold to reach the outer shell. I pointed it out during several yearly reviews and finally it was replaced. My electric bill took a sharp nose dive which was expected because the new fridge had to be way more efficient.
If your gasket is good and it sounds like you've tested all you can for this, then maybe it's the same thing. If so, I have no idea if this can be fixed.
Strangely, in my new apartment, this fridge has a hot spot near the freezer door. The IR thermometer says it's nearly 10 degrees hotter than other areas of the same side.
Here's hoping that someone here is a tech for fridges and can at least explain what's happening.
