I'm answering this a little late, but I'd say that if Maxwell did the shock biscuits, that is not the problem. My current M20C is just like you describe, the right wingtip is a bit lower to the ground than the left. I've worked on enough older Mooneys to say that while not common, it is not unique. Each airplane was built by hand, and I've always wondered if a combination of slight mis-alignments can all add up over the length of the gear mechanism. In fact, some airplanes appear to have main gear legs that are not the same angle to the ground, even though nothing is bent and everything is rigged perfectly by the book.
I have not noticed this amount of difference in later (M20J and on) models. Perhaps fewer hours and wear and tear, or perhaps better factory jigs by that point.
I'm impressed your wife could actually tell which wheel touched down first. Mine usually just chocks up all my landings to bad technique on my part.
