I don't have the mower manual right in front of me, but I recall that to "drive over and attach" the mowing deck, the manual says that you should have the PTO in the "rear" position so the mid PTO is in neutral configuration to engage with the mowing deck as it attaches. I can't remember what the manual says about which PTO position to move to once the mower deck is attached, but it seems like your are right, the mower should be ready to engage whether you have the PTO in the Middle position for both mid and rear operation, or all the way forward for mid-only PTO operation. After you have it in one of those positions, you need to pull OUT on the "engage" knob, I would call it (forget what term the manual uses), which is just to the right of the steering wheel, it's yellow in color. That will actually make the mower blades engage and start turning to cut. To stop them turning, like if you want to back the mower up while mowing, you push IN on that knob. That disengages the blades while the engine powering the mower is still on. If you have the mower blades cutting and you put the tractor in reverse, the tractor engine will automatically be killed unless you push that knob in and kill the blades first. You will soon be an old hand at all of this!
