Instant loss of charging... car is now dead on road...

I'd guess you have something like a bad connection in the bulkhead connector, the ammeter connections have "gone away" or the "in harness splice" has broken. Hey, it happens

For example, a break in the charging line from the alternator through the bulkhead to the ammeter circuit would result in high alternator voltage, and no charge.

ANY CAR should run for hours off the battery with no charging system. Hell, you should even be able to run it a FEW miles with the headlights on Sounds to me like you might have a battery on the way out, too.

First thing I'd do is get the battery charged and PROPERLY load tested.

Pull your bulkhead connector apart and inspect it

Read the Mad article: (which gives you a simplified diagram of the heavy current conductors in the harness

http://www.madelectrical.com/electricaltech/amp-gauges.shtml