This might be better in its own thread in the Sega hardware subforum (I think one exists?), but just to get the ball rolling:
1) Yes, but you need an amp
2) It might not be fine unless you have something that accepts 24KHz, because the Model 3 doesn't output "VGA" (31KHz/640x480) IIRC
3) I think you probably have to hack something
4) M3 uses +12V/3.3V/5V so yes
My personal opinion as the owner of >1 Spikeout board is that the Model 3 harness is enough of a giant mess of spaghetti that unless you really have the time to spare, just buy a used harness and hack it up, or pay someone else to do it. It's a LOT of wires.