VR,
I really can't speak to the voltage/resistor issue on the LED lamps. I just remember recently seeing a lamp that I'm pretty sure had something like the voltage range I mentioned. Darned if I can remember where I saw it. It was a specific bulb so, as I said, you can't generalize it.

6V vs 12V - Yep, a 10W bulb in a 6V system would be pulling about 1.66 amps. In a 12V system a 10W bulb draws about .83 or a 20W bulb draws the same 1.66.

VR is right about bag swing and bounce. A battery is a pretty heavy load, and it has connectors of some kind. You don't want it bouncing around. I have a 7 ah battery in a small handlebar bag on one bike. I have it lashed up pretty tightly to the bar with a nylon strap and buckle. It does not move.