In the words of Sinstar, "Beware, I live"
As I mentioned in the previous post, I started over with lr-mame2000 (MAME 0.37b5 as a Libretro core) on RetroPie 4.7.1 (current). While mame4all-pi is supposed to be faster, it doesn't do me any good if it doesn't support rotation. It also became obvious that mame4app-pi is basically an unsupported hack.
Once I started over I tried the recommended solution of disabling the internal "soundcard" without success, likely due to the same problem of the USB headset being "card 1" rather than "card 0". I then re-enabled the internal soundcard and instead configured the system so the USB headset was "card 0" - and it worked!
So this morning I made a stand out of a cardboard box (~20 degree tilt) and tweaked the libretro config to rotate and fill the screen at native resolution (along with rotating Emulationstation).
So now the task is to go through ~450 games to see what's playable and what's worth playing. After that I can focus on setting up the front end.