If you have an old unit for parts, then that is quite an advantage.
To me, if the HST was worn or needed work it'd still creep, particularly with the wheels off the ground, it just wouldn't have a lot of drive. So my money is still on something disconnected somewhere that means it's not going into gear. Particularly when the 3pt is lifting (so it's pumping hydraulic fluid), and the PTO is running (so the transmission is getting drive).
I don't know what behaviour you should expect to see if your range selector is still in neutral/otherwise broken, but the HST portion was still functional. Range selectors are usually finicky - there's no synchro and they're straight cut gears, so when the tractor is stationary they usually won't engage - you have to go back and forth a bit. Did it give you any feel that it was difficult to engage - the gears weren't quite aligned? My theory here is that if it was easy to move the range selector, that probably means it wasn't actually going into gear. Have you tried both hi and lo on the range selector? (just in case one is working and one not, for whatever reason).
To me, if the HST was worn or needed work it'd still creep, particularly with the wheels off the ground, it just wouldn't have a lot of drive. So my money is still on something disconnected somewhere that means it's not going into gear. Particularly when the 3pt is lifting (so it's pumping hydraulic fluid), and the PTO is running (so the transmission is getting drive).
I don't know what behaviour you should expect to see if your range selector is still in neutral/otherwise broken, but the HST portion was still functional. Range selectors are usually finicky - there's no synchro and they're straight cut gears, so when the tractor is stationary they usually won't engage - you have to go back and forth a bit. Did it give you any feel that it was difficult to engage - the gears weren't quite aligned? My theory here is that if it was easy to move the range selector, that probably means it wasn't actually going into gear. Have you tried both hi and lo on the range selector? (just in case one is working and one not, for whatever reason).