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

I would double check your grounds are solid. You may have fried the battery and it is no longer taking a charge. Your going to have to check the voltage without the jumper cables to see what is going on. My only guess is that somehow with the jumper cables on it is acting like a series circuit versus a parallel circuit......reason for checking the grounds. I may be thinking this all wrong. hope you find the culprit though.