At this point Spock assumes command when the bridge loses power because that's how engineering works, Captain.
Our theories of how a possible warp drive could work don't use space based motion so the ship couldn't coast because it's not physically moving at superluminal speeds. If spacetime is modified with solar system levels of negative energy in something like a Alcubierre warp drive, tuning the engines off would drop out of warp and return the ship to mundane physical universe speeds.






