Just an update for everyone - I have been able to use the tractor by jumping
the starter, after having no luck trying the jumper method on the PTO switch, which I guess means that switch is not the problem.
I've been hurrying to get a project done, but hopefully tomorrow I can take a break to try to jumper the safety switch on the transmission, to see if that might be the problem.
Regarding the shuttle lever, correct me if I'm wrong, but I don't thing I've ever had to worry about that lever being in neutral to start the tractor - just the speed selector shiftstick needs to be in neutral, right? And under the speed selector stick is the wire I think I need to jumper to test that switch. If I'm wrong on that, please let me know.
But to BigTy's point, the first day I had problems getting started, I made sure everything was in neutral.
Thanks again, everyone!