1. Shogo Amakusa developed the Rai Ryu Sen by himself. It is not a part of the Hiten Mitsurugi Ryu technique.
2. The Shimabara arc of the anime is not in continuity of Rurouni Kenshin. When the Shimabara arc was developed, Nobuhiro Watsuki was still writing the Jinchu and Rembrances arc of the manga. Being impatient, the producers of the show decided to make up crap. The show was canceled as a result of low ratings due to the Shimabara arc's crappiness and the Jinchu and Rembrances arc was never animated..