The letter
All of your theories are valid, of course. They can also be combined with Kurihara’s final theory that the letter wasn’t written by Keita. In that case, Keita and Ayano committed at least one actual murder and completed the ritual as Shigeharu expected (either out of conviction or lack of choices). Keita willingly took the fall to keep the rest of his little family safe (whether Ayano loved him or used him is irrelevant). After his arrest, Ayano wrote the letter in his name. Her motive was to make herself and Touya, whom she still felt responsible for, sound as innocent and harmless as possible, so that Yuzuki would agree to come live with them in sisterly harmony.
The chopped up body
Ok, here’s what really happened: Kurihara was right to assume that Yoshie leaked the list and the information about Misako’s pregnancy to Shigeharu, but he’s wrong about her motives. She didn’t really care about a grand-uncle who got murdered decades before her birth, she just wanted to make sure the main family wouldn’t murder any of her immediate relatives. Since Seikichi had only 3 surviving children, and some of their descendants either died, lost contact to the family or didn’t have children, there were only a handful of candidates left, and most of them belonged to Yoshie’s branch of the family.
This didn’t seem like much of a problem at first, because the chance that another handless child would be born seemed rather slim at the time of her marriage. However, when Misako called to tell her of the ultrasound, she knew she had to take action. She called Fumino (who had always seem much saner to her than her husband), in the hopes that she would convince Misako to have an abortion to end the problem immediately and keep Shigeharu from going haywire. Unfortunately, this backfired spectacularly as Shigeharu found out and arranged for Misako to be locked up instead.
Over the next couple of months, Yoshie and her husband grew increasingly afraid as they saw how seriously the other family members discussed the ritual. When uncle Kimihiko suddenly died of a mystery heart disease, they naturally assumed that he’d tried to go against his father’s wishes to save his sons and Shigeharu had arranged his death. This made their blood run cold, because he man who’d murder his own son also wouldn’t hesitate to kill his granddaughters. Afraid for her daughters’, mother’s, grandmother’s and her own life, Yoshie proposed a desperate plan: Her husband had to kill his nephew Youichi. With some luck, this would make Ayano the new guardian, which would get her off the victim list and give her full control over who got killed.
This part went as expected, except that her husband couldn’t take the pressure and drank himself to death and she was forced to marry Kiyotsugu.
Over the next couple of years, Yoshie gained a certain amount of trust by seemingly helping with the research to find all the members of the branch family. She was occasionally invited to the main house to present her findings and gradually assembled a list of people who might be either easy to manipulate or easy to kill. Since Shigeharu was every bit as gullible as his father, he accepted her input without fact-checking. On one such occasion, she managed to sneak away into the secret tunnel system with Ayano and Keita, who were locked up together while the family prepared for their wedding. Together, the three had a brainstorming session to find even more suitable candidates, and Keita volunteered the names of some of his schoolyard bullies, because he figured that if worse came to worst and they needed to commit a real murder, these would be the easiest people for him to kill.
The newlyweds moved to Saitama. The first ritual happened more or less as described in the latter, with the possible exception of Miyae’s death being a real murder. The year after that, they conveniently found someone on the internet to sell them a pristine left hand without a body attached, no questions asked.
The next year, the ritual got delayed by their move to Tokyo and they could find neither a body nor someone from the list to bribe into vanishing on short notice. So they decided to implement Plan B and go for one of Keita’s bullies, who had also just moved to Tokyo. They lured him into the house on the pretext that Keita wanted to talk to him and put their differences behind them, then offered him drugged wine and led him into the storage space next to the garage. Keita had volunteered to do the killing and had intended to use a non-messy method like suffocating the now unconscious man. But when he saw his victim defenseless on the floor, he suffered a violent flashback of the many times he himself had lain defenseless while this very bully kicked him, humilitated him and filmed it. Loosing control of his emotions, Keita grabbed a knife and mutilated the body until it was nothing more than a pile of body parts.
Of course, THIS IS ALL JUST SPECULATION…
