Based on what I am seeing and have more recently experienced, the distibutor is the culprit. Corroded, green gunk and carbonixation were on the contact points. I have cleaned and polished them and am preparing to reinstall, but this is clearly a replacement item since the erosion is so evident.
Also, I noticed that one of the six points (?) is more eroded than the others, causing a wider gap between it and the rotor. Perhaps that is the cause of persistent stumble at idle?
In any event, it died a few times on the way home, jerking and shuddering when I was able to restart it, and then conk out again. It did not behave normally until after the sun went down and it cooled off outside.
The ignition coil was nowhere near hot, and produced a strong spark (through the wire's insulation and into my hand
) when I tested it. The spark plug wires are old, but clean and otherwise in good shape.
My only dilemma now is whether to risk driving the 25 miles to the mechanics tomorrow to get a new distributor, or have the car flat bedded.
Thanks everyone for your good feedback.