The EASIEST way would be to use a J-PAC rather than an I-PAC.
If you want to stay with the I-PAC, yes you will have to cut into the existing wiring. Rather than cutting all wires and then tracing them back to the buttons, google the JAMMA connection standard and then cut the wires one at a time from the relevant pin on the JAMMA connector and connect it to the I-PAC before moving on to the next one.
Use a power strip for the computer.
Whether to connect the marquee light to the PSU or power strip would depend on what input voltage the light would require and whether the PSU could support all demands.