Make sure you set the bitrate high enough when you render it. I think the Mavic records at 60,000 bits/sec, so if you set it to anything less you'll probably start to see the quality of the rendered video go down.
2014. But a 12v car battery should be able to charge a small Mavic battery without the alternator running. The Mavic charger has built in circuity to protect against a low voltage source (the car battery) that turns off the Mavic car charger when the voltage from the car it is charging from...
It's limited to UHD 3840x2160 with the free version, which I believe is technically 4k, so I guess if you record in the 4096x2160 aspect ratio with the Mavic you may have trouble with editing the clips. I haven't tried that yet. I like the 3840x2160 UHD aspect ratio better anyway.
I store iPad Mini 4 horizontally in my hard case. I shaved about 1/4 inch of foam away in the spot that I store the iPad to make a recessed area to hold it in place. I'll try to send a photo when I get a chance.