For detection of scenario 6 (FET is shorted or failed in the "ON" state), it might be good to consider a feedback "sensor" on the heater output to identify the fault using a digital input pin on the microcontroller. In other words, look at the output when the heater should be off. There could be a programmatic "blip" in power applied at some interval and the firmware looks at the output duringby toddimus - Duet