Now there's no doubt. Evan's 100% that knight she killed in her past life. He just doesn't has his past life memories with him yet. Which he'll probably conveniently remember near the end of this story.
Now what will Serena do to Rosemary? Scare her, probably? She feels sorry for her irresponsible enabler mom so it's less likely Rosemary will end up dead by her hands.