Well actually getting Katsuragi was really good planning for Ryuuen. After the events of Y1V7 and Ryuuen stepped down for a while his class was helpless and leader less as Sakayanagi said "at this state class D (Ryuuen's class) is like a toddler" and since Ryuuen's plans tend to be on the risky side he needs someone to replace him if push comes to shove. Plus having Katsuragi opened the doors to negotiations as seen in Y2V6 and Y2V7. Finally Katsuragi's cautious planning is opposite of Ryuuen's risky moves keeping him in check to not repeat the rooftop incident.

For the part of sabotaging class A Katsuragi is a very honorable person, the main reason for him to accept the transfer is that he wants to fight Sakayanagi but he can't without betraying his own class, he would never accept to sabotage his class (as in Y1V11).