Answer to "only one drive wheel is spinning"You may have roasted the hydrostatic drive belt on that wheel
Answer to "mower deck will not engage when the PTO switch is turned on?"I had the same problem... I found one of the wires had rubbed of the plastic coating near the clutch and was grounding out. I put a shrink wrap tube on it and rerouted the wire from where it was rubbing. Haven't had a problem since.