OK so from your input and your input on other threads too (go internet!) it seems like with my PTO I've learned:
1. PTO is possibly an Independent PTO due to the engage/disengage lever being between the shift levers. A few threads lead me to believe this while a few others leave me in doubt. Just a curiosity.
Yes, it IS an Independent PTO
2. PTO should engage/disengage via center up/down lever. It does not seem to do this. Perhaps I should put a bush hog on it as next step to see how it performs under load.
Correct, lever Forward, PTO engaged; ON. Lever rearward, PTO disengaged; OFF
3. I have no left foot PTO disengage. Only a diff lock lever, to my understanding.
Correct, Lever would be where you have a plug. Top of lever would correspond to symbols of PTO cast on transmission case. Foot pedal/lever is differential lock.
4. It sounds like perhaps the PTO brake or clutch assembly is failing, which I can test for pressure somehow around the base of the shifters. I see two green bolts, two black bolts more towards the steering column. I would like to test this if I can figure out how. Between myself and my neighbor we have the tools to accomplish, I'm handy just not a mechanic and not much tractor experience.
You need to remove the steel cover that has the shift pattern sticker on it to access the test port plugs. To the LSFT of the RIGHT shift lever (speeds 1-8) is a hex plug. This plug is for the PTO Clutch test port. FORWARD a couple of inches is the PTO Brake test port. Adapt two gauges, one to each port. You are looking for approx. 140-160psi for early models, 125-135psi for later models, at 2100rpm. When you move the lever, one port should go to zero, then the other port show pressure.
Another test you can do is apply compressed air to each test port. Tractor NOT RUNNING!! If either does not hold pressure, most likely cause is broken internal line or leaking clutch pack or brake apply piston.
Photo of what I see attached. I'd love to be able to test and determine the problem without hauling back to shop. FYI the shop didn't say a word except "it has a live PTO" but the number one requirement for me to take it off the lot was to be able to go home and start bush hogging. I guess I can, but kinda irritated.