Ornis: re energy
Horsepower-seconds is indeed a unit of energy.
The kinetic energy of the rotor depends on the moment of inertia of the rotor, and the rotational speed squared. Note that it doesn't depend on the blade pitch, although the deceleration rate does.
The rotor typically stores ~2-5s of FULL engine power, so you typically have rather longer than 1s stored up, but you're right - there's not enough: you need to transition quickly to converting potential energy or kinetic energy from forward speed in order to maintain that reserve of energy.
To stop the rotor dead would require some specific choices. If it slows markedly, the the lift will be inadequate, the aircraft will drop, and this would certainly increase the airspeed from a hover, which could provide suitable airflow to get it going again, depending on its configuration.