Progress... and it's a little embarrassing! I FOUND A CLAW on my old, bent, parts Suntour. It was bent too (not sure what it should look like, I tried it that way, and no deal) but a little hammering on the vise straightened it out. The Suntour Vista is now on the Super Course, and I'm in the process of adjusting the various screws and angles. At the moment, when it's shifted onto the large rear sprocket (28-tooth) the head of the jockey wheel's axle bolt is either rubbing or is perilously close to rubbing that broken plastic spoke protector. (Earlier, the bolt head was actually catching on spokes. Changing the angle of the derailleur relative to the bicycle frame seemed to help.) I'll remove the broken plastic and shift carefully (if at all) onto that large sprocket until everything is adjusted and fitted properly.

Apologies for all the fuss, guys! Obviously I'm no bike mechanic and had never heard of a claw before.