When the microwave stopped heating I took it off the wall and opened it up. I first checked the diode based on other people's stories online, and sure enough that was the cause. A diode should have infinite resistance in one direction, and low resistance in the other. Mine was completely open both ways, clearly it was burned out. Once the new one arrived it took just a few moments to install it and attach all the covers. Now the microwave works like new!
Just be sure to safely discharge the capacitor before removing and testing the diode. As easy of a job as this is it helps to have a second set of hands removing and re-installing the microwave on the wall bracket for under cabinet installations.