Kill Switch For Car to Deter Theft
If you really want to use the USB option for the security switch, personally I would keep it simple by rewiring the USB dongle to just contain a link between two pins.
Effectively acting as an unrecognisable switch, linked to a relay.
Let the relay/s do the heavy lifting of isolating items, even in sequence if preferred.
Wiring the relay to stay open once energised would prevent everything shutting down in the event the USB was removed while driving, (or to enable the USB to be inserted and then removed before driving) Shutting down once the ignition was switched off. Although you would still be relying on the relay not failing, but that's no different to other components in the car.

In the USB I'd link pin 2 and 3 in the hopes that if it was ever accidentally inserted into a PC it would be less likely to damage it. But that's not a theory I am sure of without researching further.

Ideally make sure its never plugged into anything other than the USB port for the car isolator. (also don't plug a genuine dongle into the car USB port)
The USB and Relay would be the most obvious items to break down, but carrying a spare relay, and maybe a spare innocent looking USB buried in the boot would never raise suspicion.