Answer to: Replacing Capacitor with a slightly less capacitanceThat 1.2 uf cap is in a tuned circuit (with inductor) which has to do with frequency and should be replaced with same value. The replacement will probably work since values of components will change slightly over time but I would still stick to the original value.
Answer to: Why is my microware running when the door is open?Probably a shorted microswitch. It will have 3 terminals, one NC and one NO with respect to the common terminal. Check with ohm meter with leads removed and power off. Don't put fingers where they don't belong, even with power off.
Answer to: Magnetron or Diode or both?What is the value of the high voltage from the transformer going into the magnetron so I can measure it with my HV meter? Oven wattage is 1100. Also, what method can I use to test the HV diode since most meters that have a diode test function can not test such diodes? I assume a HV diode is a diode that actually is several diodes in series in one unit and the average meter does not supply enough voltage to forward bias the diode?