The above link is to the service manual for your microwave. On page 45, is the wiring schematic. In the center, you will find the Fan Motor listed just under the Oven Lamp. The only way for the fan motor to run is for both the Primary Interlock Switch (i.e. what you called the "bottom door switch") and the Main Relay (Lamp Relay) on the PCB to be closed. The problem is the Primary Interlock Switch is supposed to be open when the door is opened which would effectively remove the 120VAC from the cooling fan. You state the "bottom door switch" measures NC, but did you test when the door is opened? Go to page 40, and perform the test for the interlock switches. Your primary interlock switch has to be bad (i.e. stays closed no matter what the door does). Therefore, when the door opens and the Lamp Relay closes to turn on the interior lamp, it also closes the cooling fan circuit.
Also, look at page 27 and read the section about the monitor switch (i.e. the "middle door switch"), and its purpose is to blow the main fuse when the contacts of the primary interlock switch and power relay fail to open when the door is opened. So when you perform the test on page 40 for the primary interlock switch, also perform it on the monitor switch.
Also, look at page 25 and read the Note in the Line Fuse section. Namely, if the fuse is blown because of the monitor switch function, you need to replace the monitor switch and to test the power relay and interlock switches for shorts.
Given the amount of interest this problem/solution has drawn over the last 3 years, I figured I would give a little more detailed explanation of how I walked through the circuit diagram to provide my answer above, so others can apply this method to their specific problem.
Below is the circuit diagram listed on page 45 in the above service manual:
As can been seen, I have annotated the circuit diagram as follows: The red trace shows the “hot” side from the wall outlet to the fan motor. The green trace shows the return “neutral” side from the fan motor back to the wall outlet. There are only two ways to selectively open and close this circuit, turning the fan off and on, respectively. The first switch is circled in blue - the “Primary Inter Lock S/W” located on the line/hot side of the circuit - and the second switch is circled in orange - “Main Relay (Lamp Relay)” located on the neutral side of the circuit. I will call these the “Blue switch” and “Orange switch” from hereon.
The original post of the problem stated the fan turns on when the door is opened.
In order for the fan to turn on, both switches (circled in blue and orange) must be closed to complete the fan motor circuit. This is the ONLY way for the fan to turn on. As we all know from using a microwave, the interior light comes on when the microwave is running and when the door is open. Therefore, we know when the door is open the Orange switch must close when the door is open in order to complete the “Oven Lamp” (located just above the fan motor in the circuit diagram and shares a neutral return with the fan motor) circuit and turn on the lamp.
So we know that when the door opens the Orange switch closes, and this functionality is perfectly normal because it also turns on the Oven Lamp (i.e. interior lamp). We additionally know the Blue switch must also be closed for the fan motor to turn on when the door is open. However, per page 27 of the service manual, “When the door is opened, the monitor switch is closed. At this time the primary interlock switch and the power relay are open.” So in normal operation, the Blue switch opens when the door is physically open, and this opens the fan motor circuit, turning the fan motor OFF. But the original problem states the opposite is occurring. Therefore, we know either the Blue switch is stuck closed and will never open, or it closes when the door is open. Either way, the Blue switch is our problem.
This is how I determined that the ONLY problem could be the Primary Interlock Switch (i.e. Blue switch) - nothing else could explain the problem. It was the only switch in the circuit that could have been behaving abnormally.
This technique of tracing the hot and neutral lines of the circuit to isolate the situations of when the circuit should be closed and should be open can help you solve 80% of your appliance repairs.