Paul,
I already told you how a measurement disturbes the microsystem: if you measure the position of an electron, it interacts with a photon and that by itself influences the position of the electron. That's it. For a more elaborate description of Heisenberg's microscope, see here.
Furthermore, about the electron just prior to a measurement of its position you wrote this: "It either had a position and was therefore existent, or it did not." Actually, no. There is a third possibility: it didn't have a position but it did exist. This view is held by the overwhelming majority of quantum physicists. It is, in fact, the accepted view in physics. I don't believe in it, but that doesn't matter here.
Let's recall that this thread arose because I stated that there is little consensus about anything beyond the truth of one's own existence. You wrote that you were stating the obvious, and I merely pointed out that there are others (in casu the majority of physicists) who hold that what you think is obvious just isn't true.
Best regards,
Marcoen