I've heard for the first question.... that the A380 will carry extra 500Kg of water for the showers.......
Also.. the pax have to make the reservation to use the showers. Pax can be in the shower for 20 minutes... 5-10 minutes of water usage... A lightning system will indicate the stage of water usage for each pax that uses the shower. Green-Amber-Red. So if the pax is taking a shower and the amber light comes up... you'd better hurry up pal... or you will end soaked in shampoo!