It's my first opportnity to show a newbie this thread: New to CarPCs? Don't know where to start?
It's a work in progress, but it's a great starting point.
For your specific questions:
1. See the thread I posted above.
2. A video splitter is the best solution for multiple screens. There are some that accept multiple inputs and allow each screen to select their sources individually.
3. The parts you use are individual choices, but there are some suggestions in the thread I posted above.
4. I'm not the expert on installation, so I can't help you on this one. Sorry.
5. Look into a multiple-source video switch, as mentioned in my answer to #2.
Welcome to our hobby.
We ask that you do reading on any/all topics that you think might relate to the goals for your project. There's no such thing as too much knowledge.
Chances are someone has already thought of something that you have. It's also likely that someone has attempted it already, and you can find the results of that here most of the time.