I'm interested to see how you get on with it!
I'm on my 3rd Android auto kit. The implementation of the first 2 was awful and still operated the MMI in the background, so I sold them on. They were ebay purchases anyway.
I have the IA kit now which is good but not perfect. If you could get a touchscreen overlay, then it would be the finished article. Not that i'm a fan of touchscreens, but thats what android auto/carplay is based around.
The Blackvue battery is tucked away in the boot behind the access panel. It takes around 40 mins to fully charge from flat and lasts around 12 hours for my setup (4K front/ FHD rear).