Plot explanation: all possible versions [spoilers]
So apparently both the book and the series are open-ended in terms of what's really happening with Grace, and since I had too many parallel thoughts while watching I thought I'd break down the possibilities, in terms of duality (i.e. to what extent she has a dual personality).
(1) No duality at all: (1a) She's fully Grace and is just very manipulative / (1b) She's fully Mary (Mary took over at the point of her death, either by supernatural means or as a psychopathological condition) and equally manipulative.
(2) She has a dual personality and is fully aware of it, having a good amount of control over it too (so her Mary and Grace side live rather harmoniously inside her).
(3) She has a dual personality and is aware of it to an extent, but she doesn't have full control as to when her Mary side takes over.
(4) She suspects she has a dual personality, since she has flashbacks of what happened, but is in denial and attributes a lot of her recollections to vivid dreams.
(5) She is unaware of having a dual personality, which is the result of a split-personality disorder (she created the Mary persona subconsciously while dealing with the death of her friend)
(6) She is unaware of the duality and is in fact haunted by Mary at the point of her death (supernatural explanation).
I wouldn't think that M.Attwood would favour (1) or (6), but I think all are possible/negotiable.