The royal nova is the cruise ship in castelia city. Its at the docks closest to the Pokemon center
Yes, you can take the Royal Unova Challenge again.
the evening
You gotta wait till its sunset
The Royal Unova takes you on a round trip back to Castelia City. While on the boat you can fight, but you don't go anywhere.
The Royal Unova will leave as soon as you get on it. You must show up between 5pm and 8pm or it will leave without you at 8pm.
In the royal unova
Royal Unova and the ship to Liberty Island are way different ships. Yes they are both in there, but Liberty Island isn't available anymore since it already passed.
as soon as you enter castelia city it is at the first dock
The Royal Unova resides in Castelia City in Pokémon Black 2. Unlike Pokémon Black and Pokémon White, the Royal Unova can be boarded at any point in game play in Pokémon Black 2 and Pokémon White 2 as long as it's boarding time.
You can get on the unova royal at nighttime in your game , it has to be nighttime in your game time thought :)
you can get a pack of rare candies if you beat everyone on the royal unova
get the national dex and wait till the evening to board