Stabilized for now at least.
The thermister assemblies are identical looking, but probably not identical functioning.
Both have 2 2 wire thermisters (one for fin temp, 1 for air temp) and a 4 pin connector. The fridge and freezer connectors are identical.
I swapped the connectors on the control board, connecting the fridge sensors to the freezer side of the board and vice versa.
The problem moved to the fridge side, a fin reading of "HI"
Connected this way, the logic was happy, and the freezer fans turned on with the door closed.
I put em back where they belonged, and managed to get the freezer side connector installed upside down, at least partially. So now I have a fin reading of 14, and an air reading of "HI" which satisfies the logic and turns on the fans.
After running all day with the fans going, my analog temp gauge in the freezer says 5, and the frost that was starting to form on the fins has vanished.
I'll need to buy a thermister freezer 3850680020, $119.95 @ Dyersonline.com, same place I got the other parts.
And then figure out how to replace it. I'll be able to get at where it exits the box through the upper vent (fridge is in a slide) and then snake the wire down to the board somehow.
Alternatively, google hit something called "snip-the-tip.com" which seems to be some sort of repair kit that doesn't require quite as much "doing", but seems a little "iffy" to me. Anyone have any experience with this thing?