How often do you forsee yourself needing an emergency kill switch? If you are safe with your wiring and use trustworthy hardware you should never need one, and I don't think I've heard of anyone on here who's had anything go up in smoke in such a rapid and catastrophic way that they didn't have time to get out and go unplug it...
if you are really so intent on having it be a real, useful, failsafe kill switch, then 1 or 3 are your only feasible options. 4 isn't that great because you'd have to stop the car, open the trunk, get out, run back, and hit the switch. In only a few more seconds you could just unplug the opus instead, making the kill switch not too much of a benefit.
1 is a bad option however because of all the additional loss and potential noise you'd end up with on the +12v lead.
3 is at least somewhat reasonable. You could get creative with some additional circuitry and add a relay in series with the switch that would prevent the kill relay coil from being energized when the car was completely off, so it wouldn't be draining power (or wearing itself out) except when you were using the car. That would also result in the opus being completely disconnected when you weren't in the car, which you may consider good (a little less static current draw on the battery) or bad (well, no +5vsb, so no standby at the very least)
Alternatively, is there any place you could mount it, like above or beside the rear seats, in such a way that you wouldn't have to run very long power cables between the battery, switch, and opus? If you can mount it somewhere that you can reach by turning around and reaching back from the drivers' seat, then at least it would still be a relatively fast way to turn it off, without having to deal with relays or anything, without having to run really long wires, and without having to get out of the car.