Okay, we've finally, FINALLY, found something wrong.
I've pulled the wiring harness to the injectors, and the red/wt lead to #1 is brittle, with bare wire exposed in several places, all the way back into the harness. ALL 12 of the FI pins show continuity ONLY to pin #5 of the C191.
While I'm gonna hold off on celebrating until the little guy fires up, it seems to me that this problem definitely fits the symptoms.
Later: I've cut the faulty wire back beyond the damage, but I'm still getting the all-roads lead-to-#5 readings, so there's probably a short hidden somewhere in the harness or the connector. I'm not finding the part in any of the Denver-area yards; does anyone have any suggestions? - or a FI wiring harness they're willing to part with?
Later still: I spent the first couple hours of the afternoon trying to find another FI harness, and the next 30 minutes fixing the one I've got. But, like every other element in this puzzle, the harness is yielding its secrets grudgingly: The thing that got it working was the removal of the rubber top cover of the C191 connector. There was no sign of corrosion and no foreign bodies, and I didn't even tug on the exposed wires: But the next time I checked, I got the correct continuity readings. (I'll be keeping an eye on this.)
So: I'm going to solder in some replacement pieces of wire and re-test the harness; then I'll wiggle the wires into position on the engine and re-test them; then I'll hook all the wires up and test again. If all goes well, I'll be turning the key sometime Saturday morning, then turning off the engine and putting all the rest of the pieces back on.
'Wish me luck' doesn't cover it: Pray for me, dudes.