Inill - You didn't mention why we are having this discussion. Is it safe to assume the engine didn't start? At 15* you need a real good battery, well charged, to start your diesel. Hopefully you have a charger running while you are doing this. The glow plugs will have to be run for a longer time to get enough warmth for the engine to fire up. You are also probably having fuel problems because regular diesel fuel will be gelled at 15* F and won't pass through filters.
I don't know it as fact, but I expect that the glow plugs will work at 10.7 volts. Just more slowly. Do you really have an electrical problem?
Let's see; 1 volt divided by 0 ohms. That's lots of current. BIG, BIG battery. Not in this lifetime. Hint: the resistance is not zero. At 20 amps 1 volt requires 0.05 ohms. It's unlikely you can measure that reliably.
Based on your demonstrated electrical knowledge I would go back and check the wiring changes and connections you made and then, not finding anything wrong, I would change the bad switch. You could just change the glow plug switch with the start switch and see what happens.
The best switch to use is the commonly available Cole-Hersee M-490, or with a rubber cap M-626. They are rated at 35 amps and should not have a problem with engine currents. Make sure the drain hole is down.
A relay in the glow plug circuit will help, but most people don't find it necessary, and it frequently masks other problems. That said, they do put them on new engines.
Good luck.
BTW, isn't Salem the home of witches? Could this be significant here?
Loren - Did I just agree with you again? Sorry.