Moved to the Power Supplies forum.
Your question is less Mac-oriented and more PSU-oriented.
1 - Splice into a molex HDD power connector (yellow wire is 12v).
2 - Set it to 12v output for the screen.
3 - 18v for the Mac Mini. I thought this was in the manual, but I didn't see it. It's int eh manual for the P1900, the P2140 predecessor.
4 - For the ACPI cable installation, you really should read the manual for the P1900.
Your questions aren't so much dumb as newbish. They show that you have done little or no research before diving in and purchasing parts.
I highly recommend slowing down and doing some planning of your install, including wiring, PSU configuration and how all this hardware you purchased works together.
From your questions, I'd suggest doing lots of research.
It sucks. It's boring. You have to weed through lots of useless crap. It's time consuming.
But you'll have an infinitely better understanding of what yuou are about to undertake. You'll also be better equipped to troubleshoot those niggling issues that invariably will crop up.