Well, good news and bad news.
The good news is I found the ultimate cause of the problem. Suffice it to say that when the experts are stumped it can't be a good thing.
It isn't.
Just to get your mental juices flowing again, remember that the tractor started missing at the same time that the oil light came on and it stopped producing oil pressure.
Today I pulled the hydro pump. The (ball) bearing looked fine. No evidence of metal shavings in the timing gear cover under the hydro pump drive gear. Hmph. Next I checked the oil passage between the oil pump and the oil filter. Clean as a whistle. I immersed the oil pump in some oil and turned it by hand. It pumped oil like it was supposed to. Next I pulled the timing gear cover. Gears all look pristine, no shavings in the cover at all. Next I tried to figure out if I could pull the camshaft without pulling the head (I'm an amateur). Nope. So I pulled the exhaust manifold then the valve cover. Didn't see anything telling in there, although I did see that the washer thingy on top of one of the valve springs had fractured and was sitting on the top of the head:
That didn't look like it would be the cause of my problem, though. I did notice something odd: A couple of the push rods had a gap of between 1/4" and 1/2" from the top of the rod and the rocker arm. That seemed excessive.
So I hopped on my creeper and looked up from the bottom again, turned the crankshaft and... Voila!
I kept coming back to the basic question: "What would make the oil stop flowing at the same time the tractor starts missing?"
Well, the oil pump works and the oil passages are clean, so the only explanation seems to be that the oil pump isn't getting turned. But the oil pump is driven by the cam and the engine was running, albeit missing. I thought in one cylinder, but discovered today that it was missing in two.
Figured it out yet?
Hint: Turns out it's the rear two cylinders that are missing.
Hint: They're missing because their valves aren't moving.
Cause: I've never heard of this before, but the cam shaft appears to be broken right in the center, where it appears to be supported by a (now destroyed) bearing. Turning the engine confirms this: The front of the cam rotates, the back half of the cam remains stationary.
I haven't pulled the head yet but I'm afraid of what I'll find when I do. Will I find broken valves in the two rear cylinders? Damaged heads? This is looking like it's going to keep getting uglier...