So when it powers off, it goes into a sleep mode. That sleep mode doesn't require alot of power, but if you have it set to turn on if the camera detects vibrations, it uses the cars battery. Mine is plugged in all the time and it only powers on when I turn the car on or it feels vibrations. With the vibration setting, it will turn on when I open my car door. But if your power source isn't powered when the car shuts off, then it doesn't use any power, just what the camera has stored up. Hope that helps 

Yes, M3 can be connected to any brand of Bluetooth earphones for intercom. 

Yes, there is a metal piece at the bottom center that you can probably hang a weight from.  

I have not tried this function. 

It is at the bottom of the center column