@IBob on temp probe placement, I think I know why it was seeing such sharp swings when outside the water. I had the wire coming down along the inner fridge wall corner, touching it for a foot, and then the probe was pulled out an inch from the corner. So, it was both very close to the walls, and wire's contact would suck more heat away when the wall cooled down.
The spring thermometer I have mounted in there to measure the air temp now avoids this problem, and has been staying just above freezing during long compressor runs despite frost buildup on the walls.
So, I wonder how I can mount the temp probe such that it gets the real air temp without this problem..?
It could come in along the top of the hanging basket, and so not contact the wall, but I want the basket to be movable. I suppose I could make a mounting rail for it.