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There are simply two possible reasons for framing Mortulus:
1) They are both Mafia. Flower is a knight of Mortulus, and Flower is a girl; they need to frame a girl and partially clear Flower.
2) Mortulus is a town member who is about to be eliminated; everyone suspects him, and it's easy to get rid of him.
If we consider the first option, then Mortulus's accusations don't make sense. I would have him defend himself and let the knight continue to be the most effective player without being targeted.
If Mortulus is a town member, then Flower is actually trying to eliminate him, and the town is set up in a way that makes it easy to do so.
There is also a third possibility: a clever combination is being used to prevent Mortulus from being eliminated, and Flower is being framed as a result.
But I don't know if the game format allows for this. This is more related to psychological games.
And a fourth possibility: Flower is a town member, but his passive position is such that his actions look more like attempts to frame others. A town member cannot play like this in a critical moment of the endgame.
In the end, this is a murky situation, but these factors are what make me lean towards letting Flower be framed, and then we'll see what happens.
Added 1 minute later
And yes, Flower's current role is definitely not a town member.