Now that I have this one back together, I have a few more insights. The input starts with a gear on a one-way sprag. The sprag is on the counter-shaft, IIRC. This should prevent engine-braking in one range. Not sure which, and don't much care. Behind that gear on the main shaft is a wet clutch pack, and it's locked by the application of pressurized hydraulic oil. The shifter switches the routing of the oil. The countershaft is always engine-driven by the sprag, and it drives the pump. But it also always drives a rear gear, which is splined onto the output. This seems to defeat the sprag, or the purpose of it, entirely.
However we will be happy if we can just have 1:1, since we tried a few ways to eliminate the entire assembly with no success. Seems I managed 1:1 through it, and if so then we're happy. Today I begin putting it back into the tractor. In this case, start by attaching the bellhousing, then bolt it to the engine, then put it back in the frame, then lastly re-install the PTO shaft through everything.
Seems that if you were going faster than engine speed, in whichever range the sprag comes into play, the pump would spin faster than the engine speed.
I'll post again after I drive it, to report how it behaves.