I'm not sure why you are putting yourself through the pain?
The cage will not come out unless you remove the dash.
If you can't bring yourself to dremmel the radio cage then
you're not going to be putting a screen in.
Are you intending on selling the car on, without the screen, in
the near future?
Perhaps you can remove the dash without removing the windscreen
if you use a ratchet with a crosshead bit in it to remove the screws
along the front edge of the dash?
I'd advise you reconsider the mini-itx option it's really not
going to give you the processor power (esp graphics wise)
that you are going to need.
Sorry to say but I think your onto a non-starter here. The screen
you have chosen means you are going to have a hell of a challenge
to control the heater sliders. The radio cage has you dead in the water
if you aren't going to be able to modify it.
I don't understand not having at it with a dremmel. You are going
to have to seriously modify the car's wiring to get a PC in. You
will have to make major modifications to scratch build a new way
of adjusting the heating controls.
Returning the car to stock after all the modifications to various
systems will be a nightmare. The radio cage seems to be the tip
of an iceberg for you.
Have you considered mounting the screen in the cubbys in front of
the gear lever? It's too low down for my liking but it might be the
get out of jail card you need? Obtaining a new gear surround
trim complete would be fairly easy (I bet GJ has one!) and you could
then modify a spare giving you the ease of working on the moulding
in of the screen on the bench, in the warm. Then just swap the
whole trim piece.
Someone else added a screen in the lower cubby...
Good luck on whatever you decide.
C