"Yes, when I turn the key now, all the warning lights light up the way they should."
• That's good to hear, and a good thing to check once in a while till your sure the original problem is cured.

"If I understand things correctly now, the faulty headlight switch would have interrupted the charge to the alternator, which caused it to fail. So the car was running off the battery, which quickly drained and the car died. This all happened during a 15ish minute car ride."
• I don't mean to prod a dead horse here, but there is a much more likely cause that I overlooked, and (unlike the light switch wiring) does relate to your odometer gear job, which immediately preceded the alternator failure and dead battery.

Paul hit on it in your other thread where he said:

"...the problem might be the little copper strip located at the very top of the speedometer, that is actually a fuse. If that is blown, has a bad contact or is loose, that will cause your symptoms."

That's the strip you can see in the pic below from Eric Stauffer's excellent piece that led to your first success. It may have cured itself if screws were loose, but IMO it's worth checking out. [I overlooked it too, because it's not shown in the diagrams.]


--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.