1. how do you want to set it up?
you could simply connect it to the headunit, or you could also ditch the head unit and just run the carpc to the amps...
2. this is kind of missing in most front ends. i personally don't use video files at all in-car, so don't just take my word for any of this..
from what i have read, there are no FE's that truly support second screens easily that are currently in deployable form(centrafuse reports they are working on it, and Open Mobile can support multiple screens, but isn't technically out yet)
i also do not know, and don't think that any FE's support the primary screen to control the content of the slave screens..
it is kind of a hack method, but one of the things you could do is setup vlcplayer to run on the second screens, and then just use something like a kvm to control what is displayed on the screens-- if position 1 on kvm is main screen, position 2 would be the 'control' screen for the rear monitors-- being that they are composite monitors, you will also need a vga-to-composite converter.(the startech unit in the link was highly recommended by some other members-- i haven't used it, but they claim that it works very well--link).
the fe choice is completely up to you. while i currently use rr, i don't believe that it is any better then the other solutions currently available, it just works good for me(most of the time), mostly because i am cheap ..(it's like asking a group of car guys what the best car is-- everyone has their favorites...)