WT - Disconnect the F-N-R cable from the link on the transmission and try to shift the F-N-R lever. It probably will not move. Then disconnect the cable at the top where it connects to the F-N-R lever. The lever will now move and putting it in "N" will allow the tractor to start. You will find at this point that the F-N-R shift cable is frayed inside the sheathing and bound itself up. You will need to replace the cable.
Results: You were correct on all accounts. The cable inside the sheathing looked like a rat had chewed it up. I replaced the cable with a factory Kubota cable. Still it would only shift from F-N and back to F. It seemed like it would almost switch to the R but no matter how I adjusted the cables top or bottom, I could never get any movement in the reverse direction.
WT - Your wife pushing in the clutch and it then being free to roll pretty much rules out the clutch and also implies the 4 speed is engaged.
Results: Since she was in the cab and I was not, it is possible that it was in gear. When I asked her, she probably just checked the F-N-R.
WT - There is no neutral safety switch on the 4x4 shifter, it is on the F-N-R lever. That is what has to be in "N" to crank the engine.
If you disconnect the shuttle cable from the F-N-R lever and move it to N, you can start and drive the tractor as the shuttle is either in F or R. You will not be able to change direction though.
Results: Correct, I didn't mean to imply there was a safety on the the 4x4. I disconnected the F-N-R cable from the lever, put it in N and all I got was a click when I tried to turn it over. The battery is full charged.