If it were mine, at this stage, I think I’d get a few hoses made up (I realise there’s a cost to this) so I could link the left track to the right, and right to left, but at the track motors. So everything was untouched before the motors, you’d simply have the pipe work extend so right was left etc. that would eliminate everything on the track frame, motors etc.
if this test proved that the fault was not in anything on the track frame, I’d swap the hoses at the spool block and test again. Hopefully then proving the fault is before or after the spool block. I know there’s a lot going on there, but at least it’s narrowing things down.
It still feels like it could be a rotary coupling issue to me, although this has been checked I think? But they also threw two new track motors at it before checking the rotary! Which seemed like madness to me, so anything is possible.