When did Drew and Miriam join forces? (Spoiler!)

This always puzzled me. They couldn't have plotted this together from the beginning, because Miriam told Drew at the end, "You came in a little late on this charade." So here's my take on the "shenanigans" pulled off in the film. See if you agree.

1. Notes to Charlotte: Miriam started it years earlier by sending notes to Charlotte calling her "Murderess", etc., to basically torment her.
2. Miriam's torn dress: Velma did it to scare Miriam off, and tipped her hand that she'd done it during a confrontation with Miriam midway through the movie.
3. Hand and cleaver in the music room: Drew put them there without Miriam being in on it. She seemd too shocked and appalled when she saw them for me to believe she helped with that one.
4. Shadowy figure in storm and broken mirrors: Drew again. We see a silhoutte figure through the curtain which has to be him, and again, when Miriam wakes up to find Charlotte in the music room with the wrecked mirrors and bloodied arm, she seems genuinely taken by surprise. I don't think she was in on this one, either.
5. Head bouncing down stairs: Miriam definitely was in on this one, as she drops the box, the head rolls out, and we see her with a knowing, grim look, waiting for Charlotte's response. Now she's clearly in on it.

In everything that follows the two are plainly working together. So it seems to me that somewhere between the smashed mirrors and the boxed head, she and Drew joined forces. Did I get it right? I wish it had been more clearly explained in the film.