OK. So. I fixed FSX, by reinstalling a keyboard driver.
This isn’t quite as odd as it sounds, when you consider that the problem turned out to be in part of the Microsoft Visual C++ Redistributable files, which any C++ application needs installed to run. The problem is that there are umpteen of these things installed, so which one is broken? Apparently, whichever one is installed by the Logitech keyboard and mouse software. So that’s fixed. Excellent. Now I can release Raydon and Banff… Except that I can’t. This time, the problem is Drawcall Batching.