Just want to say thanks for working this out. Followed your description to get ours working similar. We have the factory reverse camera so i wired the 4TDP relay up using accessories one side through a switch and the otherside the positive of the reverse lights. This means when the car is pit into reverse the relay switches back to stock screen for the camera. Can also be overridden via the switch.
My only problem is the rgbsync converter i used does not have a battery backup onboard so it wont remember the contrast and brightness settings. I might just use an old phone charger to give it permanent 5volts to keep its memory. Im sure its current draw wont be too much.
Good because I am preparing to install a DD touchscreen in place of the factory radio and lose that all together. Just want to get a few things updated on the PC and then get a new amp and speakers for the car.