If her head snapped back at a sharp enough angle, then...

I don't think the stool was part of it at all, actually. W- Well, potentially.

If your head pitches backward, like in the way we saw Thomasin's neck broken, then your hat would come off... and Thomasin's coif is in the hallway.

T- That's weird, right? I can't think of any reason why it'd just be dropped there otherwise. I think the hallway is where Thomasin was really murdered.