Age and service life on a dry clutch are not always connected. It looks like there is a cover plate under the clutch compartment, probably access for the PTO clutch adjustment you mentioned. With that cover removed you should be able to reach the throw out bearing and the various release levers on the pressure plate. I would start there. There should be three levers for the transmission clutch and three for the PTO clutch. Look for one lever to be out of synch with the others. Sometimes the pins, links, or even the levers themselves will break or become disconnected. When that occurs, the symptom is often just what you described. If you remove a clevis pin from the external pedal linkage you can let the throw out bearing back away from the levers farther so one lever out of place may be more obvious.
Does the PTO portion of the clutch work correctly now, or have you even tried it? That doesn't necessarily mean anything, I'm just curious.