I think that it will be essential that GM provide a practical quick-release plug

Like Apple's Mag-Safe magnetic laptop power connector.

Breakaway would be cool, but I think it would be sufficient to detect if there is any impedance on the cord, and if so not allow the car to go in to gear.

The EV1, Chevy S10EV, and many other EVs have done this successfully in the past.

I would expect this to be a standard feature in the design on the car.

I'd think they would just disable the motor electronically until the plug is removed.

plug sensed = no motor is a pretty easy way. It's not like an ICE that warms up so there's no leaving it running in the garage.