I would think that your seat safety switch is malfunctioning which would cause the engine to die when your throw the PTO switch. I have some questions about the "one speed" issue. Have you done any recent maintenance (belts) on the tractor. Was it working for you before this post? Sounds like the moveable sheeve on the variator is stuck. If you lay down under your tractor looking up you will see where the two belts (one from the front, and one from the back) meet. The pulleys they meet on have a shared face. When you adjust your variator back and forth that allows the sheeve to move left and right changing the height of the belts on their respective pulley. If that sheeve won't move right and left then you have "one" speed. I recommend that you pull one of the belts and then by hand see if that middle sheeve can move left and right. If not, that's the problem! The solution is to lubricate it or replace it. First you will want dry graphite lubricant to do the lubrication. Also you will need to move it to get the lubricant on the (looks to me like brass) shaft it slides on. Once you have it moving freely it should be OK! If it won't release then you will need to replace. That will be another thread!