let me put this plain an simple. You should have the exact same voltage on those two wires at the inertia switch no matter if its 12v, or 6. The fact its dropping so much voltage leads me to believe it is the switch but again this is only if you were measuring it the way we told you and I am still not sure it is. Again you can test this easily by simply disconnect the switch and figuring out a way to connect the two wires that go to the switch. If it were me I would strip away a 1/4" of insulation a few inches down from the harness on both wires, and then simply twist them together temporarily. Cover it with some tape to be sure it does not short out and then see if the car will start. If it does, you switch was the problem and replacing it and putting some tape on the wires where you stripped them should take all of 5 mins. Dropping your tank to replace the pump I will bet will take you 3-4 hours by the sound of how this is going so far. If it were me I am lazy and would do the 10 min test before I ever started to drop the tank but that is up to you. I am out, good luck.