Ishido , trying to extricate himself from Toranaga's high stakes trap, has kinda shot himself in the foot.
I don't know how much you know about Japanese culture in the 1600s but honor and shame were huge parts of it. If Ishido fails in his sacred oath to the Taiko as lord of Osaka castle and custodian of the heir, he brings shame to himself.
In the context of this episode, what will likely happen is the Japanese nobility, after hearing about Mariko's death while a guest of Ishido, is not going to be sympathetic to him anymore against his power-hungry rival. This is HUGE for Toranaga. Ishido may try to brush it off all he wants, but his actions have alienated the honor-obsessed samurai class and made Toranaga more palatable to them in comparison..to this peasant who clearly knows nothing of honor, while Mariko is a goddess and a bushido legend who reflects very well on Toranaga. If that's who he surrounds himself with, then he must be honorable as well, right?
Ishido better eliminate Toranaga immediately or he can kiss his ass goodbye.