Why not just use a manual transmission 3 or 4 speeds and reverse in the pto line????
Size. It would be so long that it would be about the same as a two-piece drive shaft. If that would work in your space, it would be much simpler.
I was thinking of a device not too much longer than a PTO extension, that could be slipped on the PTO shaft when needed.
Another idea is 2 spur gears in a case, with one input shaft and two output shafts. If stuck, simply move the PTO shaft from one output to the other.
Bruce