Killer: Coco Chanel
Killer’s helper: Ernest Hemingway

Scapegoats: Pablo Picasso, Cole Porter
Secondary characters (if included - additional optional rooms): Salvador Dalì, Harry Houdini, Zelda Fitzgerald, Sheilah Graham

Weapon: perfume

Coco Chanel: killed F. Scott Fitgerald because his latest novel “The Great Gatsby” was a clear attack to the lifestyle of her main target customer segment. Publishing the novel would have potentially harmed her business and ruined the profitability of her latest line launch.
To avoid the novel’s threat to her business empire, Coco decided to flirt with Francis, convincing him to wear a scent she specifically produced for him. The scent becomes toxic, in absence of oxigen. That’s why Francis died inside the box and also the reason why Zelda’s hands were hurting after she found the perfume in the jewelery box.
That night she was the last one to see Francis before he went into his magic box for a Houdini style entrance. She invited him to her room and flirted with him, but Francis rejected the avances and reaffirmed he wanted to stay loyal to Zelda. She asked him to, at least, accept her gift: a scent she developed specifically for Francis. He accepted and she kicked him out of the room in a hurry as she heard someone breaking into her private area. A drunk Picasso almost ruined her plan, snooped around her bedroom and touched all of her perfumes. She rushed to hide the indicted scent in a jewelery box, but when Picasso realized that something dodgy was taking place there he stole her gun and ran away, thinking that he just saved Francis from being executed.
Finally, Coco followed him in the Magic Box room, after she heard a gun shot, and left on the ground the pianist glove that she picked up after Cole Porter show. The detective team would have found out that the pianist brought a gun with him at the party and he would have became the first suspect. Homosexuality was not well seen in the early 1900, it would have been an easy scapegoat to feed the media.

Other backstories

Hemingway: he was jealous of Francis’ writing talent and wife. If “The Great Gatsby” was to be published he would have had been replaced on the role of most talented writer of his times. Francis was younger, funnier and together with Zelda they were more than famous, they were projected to become iconic, incarnation of the Long Island lifestyle. But it was not too late. Ernest and Coco Chanel would have filled that empty spot once they took care of Francis. Zelda alone was not a threat.
The murder night he casually learnt about Cole Porter suicide plans, while chatting with him at the bar. He suddenly realized that by offering a scapegoat to Coco, he would have finally got in good with her and plan a life together.
He noticed that Pablo Picasso was getting high with Salvador Dalì and rumors came to his ears that he was harbouring revengeful feelings towards the Fitzgeralds for debts they never paid off. The plan was easy: swapping the guns between Porter and Coco and find a way to put the real gun in the hands of the drunk Picasso. It would have been pretty clear to the detective teams to suspect of Picasso and ignore the signs of a toxic scent on the crime scene. He placed Picasso’s artwork in the room of the Magic box and the artist did all the rest, under the influence of drugs.
Hemingway stopped at Zelda’s fitting room to peekalook, he wanted to see her for a last time before the death of her husband. Part of her beauty was about living life with lightheartedness. Whit the love of her life gone she could have not afford anymore to avoid responsibilities and bad thoughts. With Francis also her beauty and charme would have fade away.

Picasso: he was not in good terms with the Fitzgeralds as they were wasting money into this huge parties, trying to establish themselves as an entertaining couple living the “Long Island” lifestyle with their travels to NYC, fancy parties, surrounding themselves with arts and music. Arts which they commissioned to Picasso, but never paid as they could not afford it. However, he had no interest in killing Francis (a successful novel was his only hope to see the money back) and he was framed that night.
Salvador Dalì drugged him and lead him upstairs, into Coco Chanel room, so that he would find the gun and became the first suspect of the murder (it was a real gun because Ernest Hemingway partnered up with Coco and swapped the one from her costume with the real gun Cole Porter brought to the party).
He shot with the gun, but not against Francis’ magic box. Because of Dalì’s hallucinogens he saw rhynos happearing on top of the artwork he created for Fitgerald. Out of anger he shot against the frame, left the piece of broken glasses and the gun on the crime scene.

Porter: he was secretly in love with Francis and would have never wanted him to die. All he wanted was to get noticed by him and suicide to die in the illusion that love was reciprocal.
During the night he shared the suicide plan with Hemingway, which faked interest in his current love troubles, only to gather more information and elaborate a plan to exploit his presence at the party and cover Coco Chanel. Hemingway stole his gun and swapped with the fake copy that Coco was wearing with her outfit.