I use google maps everytime i hit the roads. I can help you with your questions. Being an android enthusiast i am glad to help you in any way i can.
The solution to this si easy as pie. GPS do not requires the usage of the phone company to work. The GPS localizes the ignals of stratosphere bounded GPS satellites and triangulates its position on real time. You can use it even when your prepaid credit is all gone, and even without the sim card.
The trick is to save the maps of the area you are traveling directly to your phone memory. you can do this in the menu that appears afeter you type a searc for a location.
I recommend you purchase a car charger or a solar power charger or an spare batery since you will travel a long way. It will continue working even when you are charging.
Voice activating directions requiere more batery usage.
Yes there is the option to save the route on your device.
GPS servies do not require aitime usage.