Rose garden storehouse

Axioms

No actual reds here, but it's affected by:

I guarantee the identities of all unidentified corpses. Therefore, there were no body double tricks!

According to Kanon:

The storehouse shutter can only be opened with a single special key. There are no master keys or spare keys for it.
This is not red but there isn't much of a reason to doubt it.

Corollaries

Shannon isn't dead in Legend

The supposed corpse of Shannon isn't hers and it also isn't a corpse. That is only her dress on some dummy shover in a dark corner of the shed so nobody would see it clearly.
Sayo is right there in the room reacting to the scene, but she's playing the role of Kanon. Eva, Hideyoshi and Nanjo are actively trying to hide this fact and don't allow the kids to get closer to her.

Leads:

One seemingly dead person working with the other servants is the most straight forward answer to most mysteries Legend throws at you, and Shannon's corpse is the only one where you could pull a stunt like this.
Not only that, but looking at the narration and circumstances of the body, it's like the whole scene is screaming: "Look out! There's some deception going on here!"
We mainly just need a way to get around the definition of unidentified corpse. We can beat the red by simply claiming that there was never any corpse to identify.

Here's an older theory requiring the presence of a 17th person whose information has been expunged from the records until the release of Turn theory.

Shannon 's corpse in legend is X Beatrice

The supposed corpse of Shannon isn't hers and it also isn't a corpse. That is simply X Beatrice trying to hide herself with the help of the adults.

Leads:

We can beat the red by simply claiming that the person there wasn't dead and thus not an unidentified corpse.
Maybe after seeing the girl from the portrait show up the Ushiromiya parents want to keep it a secret in fear for the inheritance.
There's room for a 17th person in Legend.

Obstacles:

Does the motive make any sense with the whole murder going on? Would this be the priority of a bunch of greedy but otherwise innocent people? And are the kids really the ones to whom this secret needs to be kept?
Her presence isn't required for any Legend solution, and her being here would break an interesting pattern with the episodes.

This is a fun alternative solution for the doorknob outside of just saying "Sayo did it"

Eva made the scratches on Natsuhi's door

Eva was in on Sayo's plan, she knew there would be murders and she wanted to avoid being suspected by the police, so she took some of the fake blood and made the marks on the doorknob with the scoprion charm hanging from it.