How would I use this test to determine if the fuel supply has been switched? Alsi, I don't think there should be a problem with fuel supply because when the system is OFF there is no current to valves and the default is diesel.

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'85 300td Frybrid WVO Kit

If the car starts on the wd 40 then you know it has fuel supply issues. Does not matter the type initially. The heavy sparking took place in the area of the control panel for determining the type of fuel use. Something might have happened there.
You might also get a quick check by removing the tank return line from the injection pump and check for fuel escaping when cranking. Just an easy test or two to make sure of what you are chasing. Either one only takes a couple of minutes to perform.
Heavy unintentional current may have opened or burnt something out. For example you might now have gelled vegatable oil in an unheated area because of a control failure of some sort. Sure it concievably could be a sudden glow circuit problem but your 12 volts to the plugs usually means they are working. That is providing they were functioning properly say an hour earlier.