Well no, they're not. A shunt from behind is easily capable of knocking your foot off the brake.
Not in my experience - unless they are sat on a motorcycle and you physically connect with the foot on the brake lever with your car.
Regardless, your insurance company will ask whether you had your handbrake on. If you did, you're in a much safer position.
They will ? Let us not forget that a shunt from behind is not the fault of someone sat in a stationary vehicle!
Also consider that if the stationary vehicle isn't anchored to the floor, then the kinetic transfer to it will be substantially less.
The occupants of the vehicle being hit are likely to be injured far more seriously if the energy transfer cannot be reduced and they are right in the firing line of a vehicle which initiates the shunt.