I use similar heaters for bending guitar sides. I put together a little PID controller that opens and closes a solid state relay (to control power to the heating pad) along with a thermocouple to provide temperature feedback to the PID... For about $20-30 per heater, you can have complete control and prevent overheating if you're ever worried about it. I like your solution!
Try getting a controller for the heating pads. You can then set a temperature that you want to maintain. They are fairly inexpensive and can heat quickly and then maintain the temperature you want. Helps your portable battery not deplete as quickly
The reason for quartering the power when heaters in series is because in a circuit Power = Voltage Squared / Resistance. Assuming both heaters are the same you have halved the voltage across each cell and hence it's a quarter.... However regarding the temp actually reached that will depend on the thermal conductivity & mass of the object connected. Having them out on a bench is not really a fair test even with that metal block. You'd need to try it to know for sure. But if you are still worried about melting the plastic of the battery case you could attach to a metal plate under, or on the side of the battery, occupying the whole area.
Uhm, why not go for a more dependable battery? Since you dont have the need for a big one, a little 50Ah Exide would be more than sufficient. When on the road (trail) its important to keep things simple.
If it's still really cold out then yes, that would be a problem. What I'm expecting is it'll be cold in the morning, so it'll need help first thing. But once the day gets going I'd expect it to warm enough that the rest of the day I'll be ok. I guess I'll see.
