PS - "Planet X doesn't exist under Mercury? Newton loses."
Yes - as in, you don't invent arbitrary add-ons to save a theory proven false.
But by the same line of reasoning the theory was proven false earlier when the arbitrary add-on called "Neptune" had to be invented to hold it together. Except it turned out it actually existed. The thing that made the difference was the fact that Planet X did not exist whilst Neptune did, not whether it was "invented" or "arbitrary". Otherwise the situations were completely the same – observations don't conform to the theory, come up with a planet to fix it. These arbitrary add-ons as you call them happened to actually work from time to time, so we in fact have instances in which they were the right conclusion.