Unfortunately, I think that MaryRose's endgame is to die. She has already realized that Kate is moving her rebellion far further along than she is, but can no longer stay in the childrens building. So instead, she's decided to attack Kate and John to make it seem like they were trying to bring her down, rather than join her. MaryRose could have killed them several times, but is mostly demonstrating new techniques with soot and almost training them for future battles. I bet that in the next chapter the Star Bearers will show up, and all they will see is MaryRose attacking the investigation squad. If I'm rate, her sacrifice will buy Kate a lot of trust and leeway within the children's building, and allow her to continue the fight against the Shadows House for a lot longer.
Knowing that failed living dolls become faceless, I wouldn't be surprised if MaryRose and Rosemary commit suicide when they are confronted. Neither of them wants to have the horrible fates that await them if they are captured.