The following describes the truth of what happened behind-the-scenes in Turn of the Golden Witch.
Culprit: Sayo Yasuda, who committed suicide halfway through.
Before the murders: Sayo played the part of Beatrice and gave Maria the letter. She bribed Rosa with a promise of gold, to which Rosa agreed. As Sayo was fully dressed as human Beatrice, with a blond wig, Kyrie failed to completely recognize her and simply told everyone that the "woman from the portrait" had appeared. She intentionally let Kyrie see her dressed as Beatrice to reinforce the idea that Beatrice was actually present on the island and that the letters were legitimate.
The first twilight: Sayo invited all the adults to the chapel to look at the three gold bars she had laid out. After she had them grudgingly accept that she was the true head of the family, she gunned them down, except Rosa. The chapel door was left unlocked. Rosa and the servants pretended that it was locked in the morning to give off an illusion of a closed room. This was done at midnight to give the siblings, excepting Rosa who was promised the gold, time to solve the epitaph.
"Illusions to illusions. The gold truth locks the lock of illusions." - Willard H. Wright
The second twilight: Sayo, dressed as Kanon, lured Jessica to her room and killed her with a gun before planting a stake in the bullet hole. She escaped and locked the door with her Shannon master key.
"Illusions to illusions. Illusions that have fulfilled their role do not leave a corpse." - Willard H. Wright
The seventh and eighth twilights: Genji murdered Nanjo and Kumasawa under Sayo's orders and hid their bodies such that they would only be found after the fourth, fifth and sixth twilights were complete, giving off the illusion that the "culprit" is still around even after the sixth twilight, although Sayo had killed herself in the fourth twilight. This illusion was not entirely achieved since before Shannon's death, she placed a letter claiming Nanjo and Kumasawa had died already, and herself and her accomplices had also claimed they'd been killed.
After murdering Nanjo and Kumasawa, Genji and Sayo locked the room with the letter inside, while the bodies had never been there to begin with. They then threatened Gohda into following their script. This is why Gohda went along with the story that a fake Kannon had shown up, and that the bodies were still inside the room before they locked it and went to the parlor.
"Earth to earth. Illusions to illusions. No illusion can create a corpse." - Willard H. Wright
The fourth, fifth and sixth twilights: Sayo, George and Gohda went to Natsuhi's room to get the spirit mirror. The details henceforth have not been completely confirmed, but Sayo revealed herself to George, who then shook his head frantically and refused to accept it. Sayo then picked up her gun and killed both George and Gohda, crying. She then closed the room from the inside and subsequently tied the gun to a cabinet and shot herself in the head, while leaving a stake next to her body. The resultant recoil causes the gun to be hidden behind the cabinet, forming a perfect closed room. She chose her dying position to be face down slumped over the cabinet, to give off the illusion the stake had simply fallen off. The room was never thoroughly examined, so the gun was left inside.
Presumably, before killing herself, Sayo exited the room to bring the stakes to pierce the bodies with, and drew the magic circle on the door. Afterwards she did lock it and committed suicide.
It can be assumed Sayo never expected George to accept her confession and had planned to die and kill the other two from the beginning in that room, given she had previously ordered Genji to hide the previous three bodies for later, giving off the illusion the culprit is still alive, which wouldn't be done if she intended to get out alive with George.
This is the only game in which Sayo kills herself before midnight on October 5th (apart from in 4, which happens by the result of Battler not remembering his sin.) There might not be a clear reason as to why, given she told Genji she'd plan to die in that room from the beginning, and continued with the game despite George's rejection, as proven by the fact that she still pierced every body with a stake.
"Earth to earth. No one will deny that a coffin is a closed room." - Willard H. Wright
The ninth twilight:
Rosa placed the letter on the spot Battler had been sitting, as ordered by Sayo. Presumably, at this point she thought Sayo was only playing dead and still went through with the orders.
Genji invited Battler to the study room. There he saw Kinzo and Beatrice, however both were a result of his drunkenness, as he's shown drinking before Genji arrives. Meanwhile, Rosa tried to look for Sayo, thinking she was only playing dead, then horrifically realized that Sayo was dead and there was no one else who could turn off the bomb. Rosa tried in vain to save Maria by telling her to swim away. The bomb then went off, killing the remaining survivors.