Got a multimeter? It would probably be helpful to probe the ATX connector when the PSU is powered up.
And the fact that the PC does not shut off when ignition goes low makes sense, because the M2 only sends a momentary pulse to tell Windows to shut down. With a switch, you'd have to hold it down to cut power.
With the fans spinning up, but no post, it's likely that the damage is localized to one or two components on the board - possibly something delivering 5V as that rail may be drawn upon for some of the VGA pins. I'm surprised the PSU failed with no damage to the motherboard, as it would have had to pass the current to the VGA cable.