Hi John, I was readin' about you're dilemma and it reminded me of a problem I had with my little C a couple of years ago. I was out disking a field when she bucked a couple of times and stopped. I had to put her in neutral and drag her out of the field. I tried engaging the clutch when we were pulling her and there was no movement. The engine was frozen solid. I was really upset and had made up my mind I was gonna have to tear the motor down.
At some point, I pulled the starter, and, to my surprise, the motor spun free as a bird. It was the starter motor that had seized and the starter bendix never disengaged from the flywheel ring gear. I could get that 1" of movement out of the engine but no more. That motor had gotten so hot that the armature was burned to the field coils.
I'd take time to pull the starter and see if that will free it up. I know this sounds crazy, but it would sure be worth finding out it was nothing more than that.