Back to the original question, it's absolutely impossible to access Guera Caverns after darkness falls. Using a walk-through-walls code reveals there to be nothing behind the guarded doors. The same goes for Liberte Whirlpool and Rolan's Soul.
Did you get the chests in the secret part of the Ice Caverns? If not, you can get there by heading east from the cave's western entrance on the world map.