Congrats with new Android device!
Regarding sound, did you try to disconnect AUX factory wires from the socket in the armrest? It gives better sound quality not only for Android but for factory sound as well.
You can see it in my video:
Have you connected central speaker to the android and tested it with factory radio and navigation on android? Without any installation manual it is not that obvious to connect those wires correctly. Two short twisted wires of android harness (white and blue) should be connected together, and two long twisted wires should go to the central speaker.
Do I get it right you've got the rear camera with dynamic lines? Actually Android has its own dynamic lines so you supposed to have them both now on the screen, which is not comfortable.
I always recommend this camera to the standard place (and order it without parking lines): http://ali.onl/1onP
It has Sony sensor, great picture in day and night time, wide angle, very good quality.
Regarding connection, NO need to connect ground wire, the ground is in RCA yellow video cable, the additional one may give noise/lines to the picture.
I don't recommend to use GPS antenna splitter, it may give voltage issues and destroy GPS android module. GPS extension cable to the factory antenna is the best solution. Good to know you've come to that solution.