So I think I tracked down the problem. I still had the earlier version of the FM mod present, its folder disabled with a preceding "-" character. But it seems class files can still be read if present in such supposedly disabled folders. After cleaning out all but the one, latest mod, I now know for certain that the new FM is working, I have the proper sound for the LKk42 gun, and the default skin is showing up when I enter a mission.
All is right with the Universe.
I also added to the cockpit class my differentiation between the gyro and magnetic compasses. Both were always treated as identical, where the gyro had the same lag in response and the same 30 degree bank angle limit as for the magnetic compass. Bad. Gyros are instantly and crisply responsive, and I made this one much more quick to respond. And I removed that 30 degree bank angle limit for the gyro as well.
But I need to apply further compass fixes, because the code is the old crude treatment, which causes the violent swing of the card whenever the plane's azimuth passes through 270 degrees. (Gods, but does that annoy the hell outta me!) And I might add a tilting action to the magnetic compass as the plane banks and pitches.