Thanks Tidder, that solves it.
I checked few worklogs and found many people are using USB hub in dash with PC installed in trunk, so I guess USB would also work. But still if anyone has faced a problem with extending USB or IGN or power or any other cables in between dash and trunk, please point out.
For valet switch, would it be possible to use steering wheel control switch, or should I install an on/off switch. Also would it make sense to have a switch between Battery and PSU (Carnetix P1900), this could be the ultimate cutoff switch.