To answer these questions all together:
I am using the stock battery, but as it is hardwired and it only draws power for the 10s it needs to power down when the ignition is switched off. I read that the battery does reduce in capacity over time but I'll see how long that takes before putting in a super capacitor or new battery (which are only a couple of quid anyway). I like that I can take the camera out and use it off the battery if I want to.
My install is shown in this thread http://www.fordowner...dashcam/page-14 It is worth reading the whole thread and there is some really good stuff in there to make the most of the Mobius. Basically I only needed a piggy back fuse tap, a 3m long cable which included a 12V to 5V transformer and ended in a 90 degree mini USB connector which plugs into the back of the Mobius. I added a second cable to the storage compartment on the dash for my SatNav which was easy to do as well.
I can't remember which fuse I tapped into but it is in the thread I have linked above and is ignition live, which means it is only powered when I turn on the ignition.
I don't run a rear cam but this could be an option sometime if I want to.
I used normal bathroom clear silicone, although my first attempt used stuff I had for gutter repair and it fell off so not all silicones are the same. This one has held firm for a few months now and that includes days at 35 degrees in full sun without any problem. I basically squirted it onto the back of the mount and pressed it onto the screen, wiping off the excess. Not sure what happens if I need to take it off, but I'll worry about that when I need to.