I doubt it. The energy is stored as rotational kinetic energy, which means you do better by spinning as fast as possible rather than being as massive as possible (just as linear KE = 1/2*m*v^2, rotational KE = 1/2*I*ω^2), which in turn means you care mainly about high tensile strength.
For perspective, this spinning car-mass has the energy of a fighter-jet-speed car. You could get that energy from mass instead of velocity ... from say a walking-speed supertanker.
Hard to say. We're trying to compare the instantaneous flywheel power rate with the average annual household usage.
During peak times (when a flywheel would need to discharge), by definition households are using more than average. But it's also true that less than 100% of the electricity demand needs to be time-shifted.
I wonder if cemented rubble, in a steel housing, could be used.