I tried it all. I adjusted the linkage. I checked everything I could from outside the tractor. When all else failed I tore into the tractor to see what I could see.
The attached file is what I could see. It looks like the release bearing went out some time ago. (Possibly before I got the tractor.) The stuck bearing then rubbed on the "fingers" of the pressure plate until nothing was left.
I'm still in the process of tearing it apart, but I thought I'd share with you all before I went further.
I am going to check the clutch disk to see how worn it is, and if the disk doesn't look too bad, I may just replace the bearing and pressure plate.
Any thoughts?