[Antera Caramichael]

Well, Rosa said herself in the end that she always lived in the shadow of her siblings, and that they always considered her as their young idiot little sister.
I think that we can throw away the scene in the chapel because we know that Beatrice wasn't there in the first place. Personally, I think that:

Spoiler for Theories for the Second Game, from the First to the 5th Twilight.:

Rosa solved the riddle, and announced it to the sibling in the Chapel, after that, she might have wanted to make a toast and poisoned her siblings. After that, she made the scene.
She is the most suspicious one: Her daughter is convinced of Beatrice's existence, it would not be difficult for her to ask her to make the over believe that she visited them. Kyrie might have taken the advantage in order to put the pression on Krauss ("I saw that Beatrice, so now tell me who is she."). The servants.. well they are only furnitures after all =p.

Now, concerning the Closed Room, beginning by the First Twilight: The Chapel wasn't locked in the first place. If we assume that the servant were in the plot, they lied. Beatrice didn't stated that the Chapel was closed in the first place. So Rosa took the key in Maria's bag, awakening intentionally Battler, then went to the Chapel, making lock and unlock the door (making it seem it was double locked by hiding it with her body), which explained this Red Truth: Starting when Maria's key was received, and until the instant Rosa unsealed it the next day, it passed through no one's hands!!. It isn't stated WHEN it was sealed and for how long! After that, it is not very difficult to force herself to vomit (Try to put your fingers in your mouth and pull the small thing in your throat =p).
Concerning the Second Twilight: Jessica, Gohda and Kanon went to Beatrice room, where Rosa was sure that Jessica, with her temperament, would go, which explained the letter and the state of the room. Then Jessica and Kanon went to her room.
But: Godha didn't went back directly to the others like we thought. When Kanon go out of the room, he meet Gohda, and then tells him that he will tell everything to her (Maybe he couldn't support to lie anymore to his love). Gohda tells him that he agree, and follow him in the room. Because her attention was focused by Kanon, she didn't notice Gohda who stab her in the back. After that, he stabed Kanon and transported his body to another spot. Then he went back to the chapel.
You notice that I didn't say "Kill Kanon". It is juste like Battler said: Kanon is only a given name! So yes, Kanon was killed in Jessica room... but it was because Kanon renounced, in other word KILLED his name ("I'm not a furniture anymore/ My name is...")! LambdaDelta can say as she want The only one who can claim Kanon's name is the person himself! but as long as she don't say The only one who can claim Kanon's name is "Kanon's true name", we can't say who/what she is talking about!

Now concerning the 4th and 5th Twilight: With what I said for the Second Twilight, we can assume that Kanon the person was not dead yet. Because he was severely wounded, he used his last forces to kill Kumasawa and Nanjo with a knife before being killed by Gohda and Shanon. The Red Truth They definitely would not mistake any different person for Kanon! can be solved because at this point of the story, the others servants still considered Kanon the person as Kanon the servant!