I can't counter your theory much more, since it overlaps with mine (IE, Shannon is Beatrice, carrying out the murders). The only functional difference between yours and mine is that Shannon is the mastermind in mine, and Rosa in yours.
But, lets say Rosa is the mastermind. Both her and Maria met with Beatrice in the Rose Garden, therefor excluding either of them from being Beatrice. Since Rosa met with Beatrice, Beatrice gave her (through Maria) the key to the chapel and a letter. Therefore Rosa absolutely had nothing to do with the first twilight.
But, as you suggest, the point of contention is the second twilight, where you say Rosa gave the order to kill Kannon/Jessica. This again, must at least be in part false. Whereas Shannon and Genji had over an hour to do whatever they wished (construct closed rooms, kill, set traps, place letters), Rosa still only had five minutes, starting from when Kumasawa woke up and when she returned from "meeting Kinzo". Now, which makes more sense, Genji and Shannon taking there time to murder Jessica and Kannon, or waiting roughly a half an hour and stumbling upon Rosa (since Rosa could not have made it to Kinzo's room in 5 minutes). Therefor, only Shannon and Genji could have killed Jessica and Knnon, but more importantly, they did it on their own. Any scheming would have had to been already done, and any changes would have to come from them.
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