The Story of the Japanese 00623 [Early Kamakura Period] Investigating the Truth Behind the Assassination of Sanetomo*

Let us investigate the truth behind the assassination of Sanetomo.
[Question 1: What was Kugyo's motive for the crime?]
Since Kugyo shouted, 'This is how I avenge my father,' it is safe to assume his motive was indeed 'avenging his father, Yoriie.' However, this raises questions about how he chose his target.
After all, Sanetomo was only 11 years old when Yoriie died. It is inconceivable that Sanetomo was the mastermind behind Yoriie's assassination, and Kugyo himself could not have been under such a delusion.
The motive itself seems full of mysteries, but for now, let us move on to the next question.
[Question 2: Who was Kugyo's target?]
The mastermind behind Yoriie's assassination was, by any account, Hojo Tokimasa or Yoshitoki. People at the time must have thought so as well. If that is the case, Kugyo's
'This is how I avenge my father'
can be interpreted as a statement born from the delusion that he had
'successfully killed Yoshitoki.'
Kugyo's target was always Yoshitoki, and Sanetomo may have been killed as an afterthought, for the reason that 'Sanetomo, who was enthroned following Yoriie's assassination, is also hateful.' In other words, because Yoshitoki claimed to be feeling unwell just before the ceremony and had Nakakira take over the role of sword-bearer, Nakakira was mistaken for Yoshitoki and killed.
[Question 3: Was Yoshitoki's sudden illness a fact?]
It is now clear that Yoshitoki's sudden illness was a lie. Yoshitoki knew about the assassination plot by Kugyo in advance and switched places at the last minute to escape harm. He likely thought, 'It doesn't matter if Nakakira, who was sent as a spy by Retired Emperor Gotoba, dies,' or perhaps he even thought, 'It would be lucky if he died.'
[Question 4: Why did Kugyo rely on the Miura clan?]
This is the difficult part. As for why Kugyo relied on the Miura, the Azuma Kagami writes, 'Since Yoshimura's son had become a disciple at Tsurugaoka Hachimangu, he likely relied on that connection,' but there is no way he could have gained support by killing the Shogun and relying on a gokenin without any prior consultation. It is natural to assume that some kind of secret discussion had taken place beforehand.
In other words, it is likely that Miura Yoshimura was a party to the plot to assassinate Sanetomo.
Having organized the facts this far, let us deduce who the mastermind was.