Matsudaira Sadanobu and the Shogunate

Matsudaira Sadanobu bribed his way into the Shogun's inner circle, specifically to the leadership of the roju. He used his influence to push the grieving Tanuma out, stripping his projects of funding and sidelining Tanuma himself. Matsudaira was also friendly with Hitotsubashi Harusada and convinced Harusada both to offer up his son as Shogunal heir. As for the sitting Shoguniya Haru, after losing his own children he doesn't seem to have thought too deeply about the matter and simply accepted the advice of his counselors to go with the NRE.