IIRC the 2010-2013 don't have the guide lines. Think it was CL where there was a long thread on it been awhile since I read it and didn't go through the whole thing.
I've had a Garmin backup camera for my Nuvi for many years, I spend a lot of time where there is no cell phone reception so still use a Garmin GPS often. When I bought the GX took the camera off the Taco and put it and the Nuvi on the GX. I have a 19 Lux with front and side (passenger) cameras and guidelines on the rear camera. I run both factory camera and Garmin at the same time.
Best thing I did for the backup camera is replace the reverse lights with Diode Dynamics bulbs, spendy but a huge difference and don't worry about Chinese electronics screwing up anything.
I believe the audio beep when reversing is from the sensors in the Driver Assist Package. Just got back from 2 weeks on the North Shore of Lake Superior hunting grouse and running tight trails. First thing I do when I get there is turn off the driver assist stuff, otherwise it would constantly be going off from the trees and brush I am busting through. But yes it is nice to have when in town when backing up and and someone goes screaming by behind you.