1. A regulated 12v source. I'd pull it from the PC's PSU for simplicity.
2. A regulator. 8awg is overkill, bu it will work
3. What kind of "connector"?
4. What would the purpose of the relay be?
5. You might consider 2awg for power to the distro block, just to be safe.
Also. The ignition controlled line does not need to come from the ignition switch. IT needs to come from a switched 12v source that turns on/off with the ignition.
Tap into your amplifier turn-on lead for simplicity.