I think you are wasting your time trying to eliminate the nag screen before you address the real issue.
Getting the tablet to automatically boot when power is applied is going to involve manipulating the filesystem. I don't think you're going to be able to just remount /system and edit something. That thread @Minionguyjpro
found talks about unpacking the boot image and modifying it's ramdisk and repacking the boot image. That's a lot of steps where something can go wrong.
If you decide to live with having to push a button to boot the tablet, you will probably find that waiting for the nag screen to time-out is not such a big deal.
And it probably takes a lot of work to make the tablet shut down when you turn off the car. So, unless you go a long time without using the car, the tablet is already booted. At worst, you need to get past the lock screen. I haven't seen anyone talk about that. Maybe you just disable it in Android Settings.