At this point, Utsuki has already joined with the wicker chest. She gets inside it with her sister's dead body, and becomes a Kuon sacrifice. Because of this, for certain reasons their consciousness fade in and out, over and over. At this particular time, Kureha, resenting Utsuki, is the stronger consciousness, and ends up throwing herself off the cliff.
As previously written, Utsuki and Kureha have already merged, but they each believe that they are themselves. But sometimes strong, intense memories rise to the surface, and they see visions. This scene is to show to the player that Utsuki isn't her normal self.
Utsuki has already merged with Kureha, and she is both physically and mentally unstable. Because of this, depending on the timing with which you meet up with her, she will switch between Utsuki and Kureha.
She is one of the women who lived at Yorichika Manor as a servant. The manor becomes filled with creatures as the illness spreads through it, and she is driven mad by fear.
This message was written by the long-haired woman who only appears in Sakuya's chapter. She was attacked by monsters and was fortunate enough to survive, but she wandered the mansion, losing blood and her consciousness fading, before finally dying. (She is lying on the ground by the shrine in Seimei's chapter.)
She was taken away and used as living bait by Kureha. When those with the sickness enter a wicker chest with beasts, bugs and the like, they lose their minds. As revealed in Seimei's chapter by Utsuki, those who are sacrificed by magical means develop a desire for flesh, and attack any nearby living beings.
This chest has been washed along from upstream (the eastern side of the manor), and happens to come to a stop at the shrine. It was being used by Douman, and probably has someone from the manor inside it. When Sakuya comes to the manor, chests are scattered all over it according to Douman's plans.
This is to represent that it is summer (the fifth month on the old Japanese lunar calendar). There are lots of irises that Utsuki took care of with her sister around the shrine residence.
These marks were created in one of Douman's experiments, testing out the power of his spells. In order to confirm the extent of his powers, he used the manor's residents and those with the sickness in his experiments, without feeling any remorse.
This is a nobleman who became a Kuon sacrifice, rendezvousing with the ghost of a woman, unaware of her death.
Doukai, engrossed in the documents collected by Douman, was killed by the creatures let loose by his master. Douman had brought him along as bait all along, catching his apprentice in a trap.
This is spoken unconsciously by Utsuki. Their consciousnesses are mixed, and deep down Kureha is calling this out.
By this point Utsuki is no longer Utsuki, and she is driven by the instinctive desires of a Kuon sacrifice.
Even Douman, who was researching the Kuon curse, didn't know how what would happen at its end. In an attempt to get ahead of the other diviners in the city, he tried to hurry the curse to its conclusion. When Seimei tries to stab Utsuki, Sakuya ignores the horrors of the curse and pleads for Utsuki's life. Thus, the Kuon chest is complete, and the reborn Utsuki who emerges from it is taken away somewhere by Sakuya. Without Sakuya knowing the true terror of the curse...