I think on the AV out the cameras you can preview will be much the same as when in the APP, but in the APP you can change what camera to preview, i am not aware you can do that with a button on the camera too.
On my SG9663DR camera you can do that on a button.
I can not recall if the camera recall what camera is selected for preview the next time you start your car, it could be you at least one time have to select what camera is the preferred preview one.
Any LCD screen should work with the AV out, lots of cheap ones of those on ebay and amazon i assume. i dont have one or i do but it is intigrated into my DIY night vision goggles so cant use it for testing.