If you can't jump the starter across the posts, and the starter runs directly off battery, it would say that the starter isn't either powered or grounded. If you have 12v from the starter post to frame, it would tell you that frame is carrying ground. You may have a short directly to ground in the system, or a ground that you can read with a meter, but won't handle a LOAD. Remember that continuity needs to be checked with a load. You can read good continuity, or even good voltage, but they could both die under a load. Use a good jumper cable and run the jumper from the batter positive to the positive post on the starter, check for power.
At least it sounds like your issue is main power, and main power or ground with a LOAD. Start bypassing with jumpers until you can isolate to either a main power or main ground under LOAD. Your PTO is mechanical engagement, and the safety switches don't effect dash power. If you have no dash power, your problem is up front.
By the way, LOAD test the battery...
I took the starter and solenoid off 2 nights ago and hooked it straight up to a battery and it spun and engaged correctly. Last night I did exactly what you are saying. I took a fresh battery and put it right next to the starter. I had the tractor battery totally unhooked. I took a jumper cable wire and hooked the positive straight to the starting side of the solenoid and the ground straight to the battery grounded on the bolt head of the starter. The starter did spin, BUT for some reason it never engaged the flywheel.......??? I could hear it spinning but never heard it or felt it engage the flywheel...