Can't hardly see the diagram but maybe i can throw some stuff out there that might help..
Yeah, the USB for the Lilliput is for the touch function.
On the drawing, there's no power to the monitor.
It looks like you want to use an Opus 250, how are you going to connect it since you are going to use the whole computer case? Will it fit inside the case?
You can tap into the cable you already have (as long as it's thick enough) and i would use a distribution block.
Where are you getting the power for your 4 ch Amp?
Never heard of "soft power" either
Wiring harness for what?