Check your voltage while the car is running. Then while it's running unplug the glowplug harness that goes from the relay to the glowplugs. If you suddenly gain voltage you have isolated your problem. Sticking relay keeping the glowplugs energized. No alternator will be able to keep up with that current draw and yes, you can cook the alternator in short order. Fairly common.

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'70 F100 shortbed
'82 Diesel Westy
'83 Euro 300TD
Curtlo Viper
Yeti ARC
Surly long haul trucker

Thanks! That tells me what I need to know about the alternator getting fried!
Yes, we pulled off the glowplug harness while checking the voltage, and the massive drain stopped. We had also noticed that the battery wasn't charging back the way it should (pointing to alternator) and after driving it a bit and rechecking alternator (with glowplug harness unplugged, of course) it wasn't charging at all... alternator just dead, and your post confirms that possibility.

Obviously the battery is OK since I was able to drive for about a half hour without its getting a charge, with lights for a part of that time (didn't test my luck for further driving). And this morning all the electrical but the starting is nearly perfect (power windows, lights, etc.)

vstech, I already replaced almost all of the fuses with good copper ones, and always check for corrosion when things go amiss. But I haven't gone back in there since these posts to recheck the fuse for horn and back-up lights.

Yak, thank you for the informed thoughts, I will check out the wiring if I need to. And once I get the car going I will check the fan, though the engine heat has been stable in the past few days through all this.

I am going to borrow a working gp relay box from my friend (while I look for a replacement) and replace the alternator when it comes in, hopefully in the next few days. I'll get back to you all to let you know the results... Thank you very much for all your very helpful replies!

OK, all is well. I replaced the fried alternator, and also the glowplug relay. Now it's charging up perfectly.

The horn and back up lights was Indeed the fuse 14.... I'd replaced Almost all of my fuses with the new copper ones except for a few, including that one. I took it out and with sandpaper got the corrosion off and it worked, tho I am definitely going to replace it.

Thanks for your help, everyone!