If you disconnect the mains then the power will be backfed through one of the circuit breakers and back out to others. If the breaker you plugged naughty plug into is off, it’s isolated. Otherwise, it’ll power whatever circuit breakers are on on the breaker panel.

If you don’t disconnect from the mains you’ll kill a person working to fix the electrical lines for your neighborhood.

Yes, safe and automatic things exist to switch automatically - or even hook up a generator in a pinch manually but safely (no naughty plug needed).

Defo don’t backfeed your abode unless you’re gonna die due to some major emergency!