Seems to be FIXED.
i replaced the 0.6 ohm plug. The new NGK part tested 1.2 ohms like the other two old ones. The tractor started without hesitation.
I broke the threaded top of one of the old plugs (I replaced the wire C connector on top instead of under the bar that connects the plugs, and I think I overtightened into it), so I'll replace that one. But I managed to wedge the stub of the broken top in place, and the tractor started a couple of times, so I believe it's fixed.
Thanks for all the help. I learned a lot!