[Grippy]

Alrighty, to answer your questions best I can.
I grabbed power from a random 12v wire on a plug in the tailgate, the power needed is minute, I stripped it back slightly and soldered it on, didn't need to worry about the earth, it all works without it.
The buttons are correct I think, go through the canbus settings and make sure you've selected your correct year and model.
For the other apps, the stock music app sucks balls
I installed Gone Mad Music Player, I played around with a few and liked that the best, it's clean and fast.
Also if you don't pay for Spotify, delete the stock app and install a 'ad free' version.
I mainly use Vanced YouTube Music, I like it much better than Spotify.
Install Teyes Widget Updater, it will link you to Music Proxy which then allows the stock player to be pissed off and the mini player show whatever app you're using, ie, any music player.
You will have to play with the EQ and other settings for a while to get the sound you like, I've found mine cranks now and sounds bloody awesome, heaps of bass from the little sub and the front doors.