With the EGR and evaporative emissions system capped off, the car started right up and fired on all four cylinders.
When the throttle is opened the engine still stumbled, but it ran smoothly on four cylinders most of the time. It would occasionally stumble but it would recover and return to running smoothly. I backed the car down the driveway and then drove it up the driveway without trouble. The last time I tried that, I had to slip the hell out of the clutch to make the thing move under its own power. Massive improvement, power-wise.
"Okay," I thought. "So I do need to replace the vacuum lines, eh?" I reconnected the EGR and the engine exhibited the same behavior. I let the car sit and run while I checked and filled the Jeep's tires, and puttered around a little; until the cooling fan switched on. I heard it come on and went to look at the thing; and there was still no "CHECK ENGINE" light illuminated.
So the computer thinks everything is A-OK, even with the evaporative emissions system disconnected? Okay, whatev--not my problem, really--and I decided that was about as much as I wanted to do today. I think I need help from someone with broader experience than mine.
...I tardily realized that I really ought to try reconnecting the evap system to see if the engine ran like it did before, so I reconnected it and restarted the engine...and it ran exactly the same as with the system disonnected.
Ergo: no vacuum leak.
However: I recalled that when I checked the compression, I replaced one of the spark plugs. It had obviously been dropped; the electrode was bent to shit. I hadn't run the car because the battery was short on power (after cranking the engine for the compression check) and I'd been tired and cold.
So here's what I think: the bad lifter made #1 misfire a lot. Sometime after I replaced that lifter but before I ran the engine again (perhaps when I was trying to do a leakdown test with half-assed tools) somehow the plug got damaged and I didn't notice it, so #1 continued to misfire.
When I did the compression test and the leakdown test, I noticed the #1 plug was bad, and replaced it, but didn't try starting the car due to my own fatigue.
So: there's nothing mechanically wrong with the engine. Electrically appears to be another story.
I'm going to try replacing the balance of the spark plugs. Maybe the plugs are bad? I'm also going to do a fuel pressure test.
I don't know why the engine stumbles when I step on the gas (and occasionally when idling) but it's the only thing keeping the car from being 100% drivable. The only thing. And in fact if it had insurance on it, I could drive it now: the stumbling is more annoying than anythnig else.
So I guess the next step is insurance, and then calling the guy I worked for in 2005, because I need someone with more experience and knowledge than I have to say, "Look, dumbass, you forgot to do A, B, and C. Here--" *click* "--now it runs perfectly, see?" Me: "OMG I'm an ass."