youre overthinking it happy. you dont want to use diodes for currents over a couple hundred milliamps.
my setup can run just the pc all by itself with just a push of a button on my keychain remote starter fob.
the remote start has about 7 different low current signal outputs, but im only showing the two that turn the pc on here. first, the ignition output acts just like your ignition key wire. i chose to use the signal wire from the 791xv so that way the pc will turn on with the engine when i remotely start it. alternatively, i can just use the keychain AUX button and the pc will come on, without anything else. and, should i be in the car and i just want the pc and the speakers on, i turn the key to auxillary position, that turns on the speakers/radio, then i hit the aux on my keychain and the pc comes on.
for the winter, or when things go weird, i use the kill switch to keep the pc from powering on when i dont want it to. in this case, i could use another output from my remote start to make this more convenient, but for now this is fine. this is a simple drawing, you should have diodes on the output of the 791's signal wires... and across the relay coil.. but basically its just that.