The rationale for
disabling the 12V is :
"Its 12 volt battery operates the SRS, airbags, windows, door locks, touchscreen, and interior and exterior lights. The DC-DC converter in the high voltage system charges the 12 volt battery, and the 12 volt battery supplies power to the high voltage contactors, allowing high voltage current to flow into and out of the high voltage battery."
If the 12V battery itself is gone in this collision, there's no low voltage current to power those above components even if I reconnect the First Responder Loop or if I short the 12V wires.
Now that the 12V is gone, there's power to power the DC-DC converter and there would be no 12V coming from that converter either.
Also, for the high voltage, as
Electrek pictured for a Model 3: There are multiple redundancies to make sure it got cut off in a collision:
1) Negative Power Switch (I think that's a contactor)
2) Pyro Disconnect
3) Positive Power Switch (I think that's another contactor)