This issue wasn't nearly as complicated as I made it. Over the past two weeks, I bought new springs, oiled and greased everything, centered the jets, added a new foot pedal return spring, adjusted the mixture, balanced the carbs, and on and on . . .
Here is the primary cause of all of my problems with a racing idle on my HS6 SUs:
Somehow the linkage must have slipped after getting hot (this started after a long idle where the temperature approached 190 degrees or higher). In the picture you can see the lever fastened with a pinch bolt fitting, and that lever, one on each carb, has a small barrel stud marked in RED that fits in the "saddle" of the throttle "lost motion lever" which is Moss part 372-840.
In all my maneuverings and adjustments, I balanced my carbs by loosening the pinch bolt, and when I re-tightened the pinch bolt I deliberately set the barrel at the bottom of the saddle so that there would be no delay or "play" when pushing the gas pedal. The problem was that the barrel was not allowing the throttle levers to return fully closed.
Last week I thought I'd fixed this by floating the barrel smack in the middle of that saddle (the lost motion lever). I thought that fixed it, but it didn't.
Today, I noticed the problem with my idle racing at 1900 rpms. So, I loosened the pinch bolt, rebalanced the carbs, and then made absolutely certain that upon re-tightening the pinch bolt on the levers (Moss part 371-860), that barrel on the pinch bolt was pushed to the very top of the saddle in the lost motion lever. That way, the barrels push the butterflies shut. In other words, the springs pull the levers up against the saddle, forcing the throttle levers to close fully. The idle is where it should be.
You can see that in the picture (which I edited for clarity after my initial post).
You'll note at least three levers in the Moss drawings: Throttle levers, levers next to the throttle levers, and lost motion. I have a hunch that "lost motion" may tell the story of what I went through.
Sorry for the long opera. I hope someone else will benefit from the above.