If the damper is sealed up with silicone that's where I would start. if the damper door is broken it will allow to much cold air in the FF section. I know you said all the thermistors were in specs but they are cheep enough to replace all of them.
The brackets were removable on both fan motors, I swapped them over and everything fit. I would hate for the customer to have to replace his old freezer because of a NLA fan motor. Thanks PDuff the Part number: 482731