Are you saying that with the borrowed control valve, the transmission regained its forward direction temporarily?
But then stopped working forward again with the borrowed valve still in place?
Have you returned the borrowed control valve to the donor truck and if so, does that truck now have both direction capability of has it too lost its forward?
If the problem stays with just one truck, I would believe the trouble to be stripped splines of the Forward Clutch drum to be the problem.
Have seen those strip out and then resume operation (but usually for just a few feet) later.
You cite having done some fairly major stuff already but have the clutch drums been taken out and inspected?