What's your goal here? (More capacity for endless showers, or monster-tub fills?)
Isolating a HW heater isn't a huge problem since it's also protected by a temperature & pressure relief valve. An expansion tank on the isolated system would also limit stresses from normal heating without spitting a bit of water out the T&P valve.
Closing just the cold inlet to the tank is sufficient to stop flow into & out of that tank, but it can't "drain the tank as hot water was used" with the cold water feed closed.