Surely you can power the control electronics from the flywheel, right? That should always work except when the thing is stopped, which is a safe state.
in failsafe systems it's best not to depend on the thing you expect to fail to behave in a particular way when it... fails, redundancy is the key.
The grid is the primary power source. Flywheel when no grid. Battery would be third. It might be enough to have a passive resistor load for the third option. But at the scale of these things, a battery to run a few hours of controller is not a big deal.