I would think the drive gear should be like the part Ronald showed, not a hole with a slot, but a solid shaft with a slot cut thru it. The solid tab on the end of the pump should interface with this slot. The fact that you could not turn the drive gear with a screwdriver in the slot would indicate that the gear train is in tact. IF you crank the engine with the pump removed, you should see that drive gear/slot roate, but if it's slot is not like the part Ronald showed, it may need to be replaced. If the pump shaft is not a square tab like in the picture I linked to in my last post, the pump also might need to be replaced.
Do you have access to a digital camera? A closeup pic of the drive slot and the tab on the end of the shaft on your tractor would be worth a whole lot of description.
If the corners are stripped off of the tab then it is probably rotating in the hole of the drive gear shaft as you suspect. Perhaps this hole was created by a failure and it originally looked like the part Ronald showed. Did you find any loose metal bits in the drive or around the pump shaft when yu disassembled it?
If it is a problem with this interface point, the only way I see this happening is if it were machined incorrectly, or if this coupling was put under excessive load. Have the tractor hydraulics ever been "dead-headed"? A Quick Connection in the hydraulic line opened while the engine was running or the tractor attempted to start with a QC disconnected? This would cause a complete restriction of the flow. A complete loss of flow delivers tremendous loads to the pump and drive assemblies. Enough force to stall the engine, blow out seals in the pump, crack the pump housing or damage the drive.
Post some pics if you can.
