Just a quick question... When the tractor would not move at all with clutch out. Did the engine stall?? With a good clutch, in first gear and engine at full throttle.. The tractor will easily drive through the applied brakes on a gear drive.

I had a loose and cracked fork in my 126. It acted very similar to what your describing. Sometimes the loose fork would allow full engagement of selected gear and sometimes nothing. Other times it felt like i was getting third gear when 1st was selected. Sometimes it felt like it went perfectly into reverse then would not move.

Not saying this is your issue but it sure sounds familiar to me

check your driveshaft pins, they shear sometimes

So, I had just a few minutes at the tractor today -- Jacked the rear end up - the tires spun around with a little bit of effort, but the left spun much easier than the right, so if the brakes are stuck it would seem to be one wheel more than the other. I sprayed everything (brake calipers, linkages, etc) with WD40. Didnt seem to make any difference. But now I can not press the clutch pedal all the way down to the lock AND it returns to up very slowly, weakly and sometimes not at all ---I assumed there was some kind of spring to brink the pedal back but I couldn't see any springs on the tractor or in the diagram that was posted

the tractor did not stall

But it seemed to be under more load than it had been - and in 1st and reverse it seemed to have very little torque - I shifted to 3rd and it still seemed to have low torque but the tractor started moving forward very slowly a friend started pushing me and within a few seconds the tractor starting moving forward. But even though I was in 3rd it went with the speed of more like 1st gear. When I got the tractor back to the parking area I shifted into R and it went but was half normal speed then tried first again and it was also like half speed ---tractor felt as if it went from a 12hp to about 6hp. But that is only what it felt like I am pretty sure nothing actually happened from an engine compression point of view

Ok. as stated before. check pins in driveshaft. They may not look it but one is probably broken

And for what it worth,
WD40 is a water dispersant.
Facts: https://www.wd40.com/myths-legends-fun-facts/

The return spring surrounds the clutch adjusting rod.
Also the main spring aids in that as well.
Check for broken springs and mis-adjusted clutch.

I will check the driveshaft pins and springs next time I am there --- could any of those affect why the clutch pedal can not be fully pressed down now ? or is that more likely the brakes since the last part of travel on the pedal is for the brakes ?

I have only ever worked on hydros before - how free should those back tires spin when jacked off the ground ?