Let's make sure we are on the same page
1)Did you test the thermistor from the mainboard using the ice water test?
2) Did you perform the heater-defrost limit test from the mainboard as well?
There are only 4 things that can affect this circuit....board, heater, limit and thermistor. If your heater, limit and thermistor tests fine, and considering you are having an issue with your display, you may have another bad board......all subject to connection/wiring problems
also and I'm not sure on this but does the instructions for your board require cutting a thermistor wire for bottom mount fridges as found on the wr55x10942 boards?