I don't know about Keine, but I wonder if Mokou's hair would ever grow back after being cut since her body no longer ages/grows.

If her body is ment to "Restore" itself to its original state at the time of she took the Elixer, then it would. If the body doesnt percieve hair cutting as "Damage" then the next time Kaguya and Mokou fight, her hair would probably regenerate upon revival.