My take on the ending

Grace was the unreliable narrator in this so everything she told the doctor was to save herself. There is no split personality, the key scene is when the Jeremiah asks her to go away with him to America and become a con artist like him. He is basically a fraud and is not a hypnotist so all the "Mary" personality in the hypnosis session is all made up between him and Grace. She was manipulative and knew exactly what she wanted. I love Sarah Gadon, very underrated actress.