Hi, I'm trying to do the same, I have replaced the original screen with the display and touch screen you suggested and as it's for a Rover 75 I had room to add an extra box to put the driver in. It will save me some messing about if you could tell me how you got the power for the driver.The problem is not the pi.
I designed my own pcb which is kocated in tv module housing.
It sends hdmi out to a touchscreen in bordmonitor.
It switches between 3 sources (oem pic from nav, pic from pi if present, pic from rearcam if reverse gear selected).
It is designed to work with and without a pi. If a pi is present the shutdown is simple with petes pibus.
But how to manage shutdown if pi is not present. How to tell the used TI1501 ibus ic to go to sleep mode and cut off pin8.
Thought about consumer shutdown (16 minutes after closing doors this line goes to 0V) but that means to pull an extra wire.
It was better to use a logic which can interprete whether ibus is active or not.
Aerial power cable might also be an option, but if you turn of radio, will this go to 0V while still ignituon is on? If so, then all the rest won't work anymore.