the whole decoy idea seemed pretty pointless except as a twist to the movie. Like why would he need to lie to his own League? It would make sense if Ken Watanabe was Ra's, then died, and then Liam Neeson (who already had a real name, Henri Ducard) inherited the name. They already stated the name is passed down to make it seem that Ra's al Ghul is immortal. Plus that solves the fact that he is Irish rather than Arabic, and also makes him a Legend and parallel to Batman.