Thanks to everyone who replied. After working with a particularly diligent Apple Support specialist who would not let go without a solution, we discovered the problem in a security setting. And, yes, Les, you are quite right, that only the pencil works in sidecar. Which made the iPad useless for my purposes anyway. As for Duet Display, I'm not going back there. After a week of troubleshooting with their support, we still could not get it to work. So I returned the iPad.
Well, at least that keeps my equipment down to only one machine -- my MacBook Pro. And I have all those pistons and rocker tabs on my stack to use for convenience someday when I sit down and figure out how to program combinations.
Right now I'm spending my time/effort learning to play with my feet and enjoying the organ instead of sweating over it. My latest find on YouTube has been Rachmaninoff's Prelude in G Minor performed with the Caen sample set, and I actually found and purchased the sheet music, too! Loving it!!!