Final Trial Discussion (SPOILERS)

[Perogi]

I originally posted this on some guy's guide but the comments only allow for 1000 characters so it's here instead. What follows is a commentary on all the cases as you encounter them in the trial and my thoughts on the "correct" answers. Some speculation on future stuff is included at the end.

It's fascinating that there are different routes to arrive at the same correct convictions, but here is my thought process:
Henry's Escape: The Witness's plan was never meant to be perfect, it was tenuous. He setup the psychic scream emitter in the hopes it may manufacture a suspect. This was the only element of him trying to encourage Henry's escape. He could not have been involved with Akiko's elaborate conspiracy involving thermite and swapped guards. He had no authority to promise her things on the next island, Carmelina did. Therefore his sound-gun setup was like the rest of his plan: an imperfect threat.
Henry's Demonic Possession: Obviously Yuri
The Murder of Grace: This was clearly done personally by Carmelina, as indicated by the evidence that she was the only one with the opportunity to commit the crime that also had a wedding ring at the time. Carmelina facilitated this murder by using Akiko, as she did for Henry's escape.
Holy Seal Marshals: I don't know why you mention Lydia, she very clearly did not do this. The tire tracks on the roof and ensuing evidence (rappelling rope, maintenance hatch broken with her knife) indicates she bypassed the first seal entirely two days before the murder of the council took place. This was very clearly Akiko fabricating the murder of her (fake) guards.
KHX Missing: As you said. Crimson is a clear red herring, KHX was in love with her and she copied the key clandestinely, no reason for her to murder him. Yuri did it at Carmelina's behest.
Second Seal: This one is interesting, both Yuri and Lydia broke the second seal as part of two separate conspiracies. Lydia learned the secret of the seal by using the remote access on her tablet given to her by Witness, who got it from Crimson. She used this to transport the demon-crate into the council chambers two days before the murders. Yuri accessed the secret physically after killing KHX and taking his key.
Third Seal: This one is weird and I made a separate save here. But basically two teams breached the third seal at two separate times. Lydia and Sam (maybe together or maybe separately) breached the third seal using Kafka's blood, stolen from Doom Jazz, two days before the murder to smuggle in the crate. Yuri and Dianon breached it using Dianon's blood on the night of the murder.
Fourth Seal: Same as before, Lydia and/or Sam breached it with stolen godflesh they got from Witness (who hired them) two days before. Yuri and Dianon breached it using the godflesh that Carmelina had fused to Dianon.
Dead Killer Demon: This was very obviously done by Witness.
The Crime to End All Crimes: I agree that Dianon killed all the council members. I believed that the demon came out and was killed by Monserat before Dianon ever arrived but I suppose the demon coming out in the middle of the fight is possible.
First I believe it is clear that Witness's conspiracy failed and Carmelina's was successful. Montserat shot two bullets, one into each of his attackers. Whoever he fired the first bullet into must have died, otherwise Montserat would have been killed and not been able to fire a bullet at the second attacker. The demon was found dead at the scene, whereas Dianon was found alive long after the crime had taken place. Therefore, Dianon must have dealt the fatal blow to Montserat after the demon was shot. Dianon likely killed the rest of the council as well given the blood traces on his knife. The demon probably did not kill anyone. The game corroborates this induction by laying out that the Witness's motivation was to scare the council, whereas Carmelina's was to kill every single one of them.
The game makes a thing about timelines and there is a bit we can infer from them. When the Witness took his five minutes phone call from Akiko, Carmelina slipped, into the corridors and set Dianon on the path of murder. Afterward she returned to the Witness's apartment, and witnessed him send the code to open the demon crate from his cellphone. How much time elapsed between her getting back and Witness sending the code? Enough time for Dianon to traverse the seals and be mid-murder? Perhaps a more assiduous examination of the timeline could reveal how long it would take for Dianon to traverse the seals after Carmelina returned, and how that matches up to the time when Witness opened the crate, the game does not make it clear. Regardless of when the demon was released, I think it is clear that all murders were committed by Dianon and that the Witness's conspiracy was a failure.
The Conspiracies: As I've laid out, there were two separate conspiracies: One planned for decades and driven by pride, and one planned at best ten years prior and driven by zealotry. One successful, and one aborted. The Witness convinced Lydia and Sam to join him in exchange for their freedom, to scare the council and convince them to focus more on the resurrection of the gods. His plan was never meant to kill, that may have ended up being a side effect, but his main goal was to scare. Carmelina planned for decades at least, maybe longer, weaving her plan into the very creation of Island 24, to kill the entire council and usurp their authority; she wanted to reclaim a birthright she thought she had been robbed of. From early on she enlisted Akiko, Yuri, and surreptitiously even Eyes to give her an assassin-child. Carmelina had better planning and more hate, and her conspiracy was ultimately the successful one.

Other Thoughts:
I suspected Doom Jazz for a long time due to the chalice and his relationship with Akiko; who else has access to all the council members' blood? Fortunately, Yuri confesses to taking the council members' blood from the crime scene and feeding it to Henry, absolving Doom Jazz. This also plays heavily into the idea that Akiko was working with Yuri and Carmelina, and not with Witness.
Is Akiko trying to bring some of her marshals to the next island? Why save Captain Voth by swapping him out for dead prisoners if he's going to be killed in a day or two anyway? We never learn his ultimate fate. You could make up some lies about honor and soldiery like she does, but there would be honor in dying for a grand purpose (conspiracy). I suspect Akiko has a plan to smuggle some of her favorite marshals to Island 25. Perhaps she has even done it before.
There is some text in the game (I think from Doom Jazz) suggesting some clues lay in the Deep Mine, but that the only ones who can grant access are already on Island 25 and are out of reach. The Judge also hints that some conspirators may be off-island, but to ignore them for now. Were these two people involved? Could they be the two that we see in the bar on Island 25?