I have to read that part of the book once more, but when I read it some years ago I was quite sure that Mr. X indeed existed. I was said that she spent beautiful hours with him side at side (intimate hours) and that he was a doctor.
As we know Marie enjoyed the society of doctors, in earlier times she had wanted to become a doctor herself, a dream, which wasn't possible because of her extraordinary status.
When I looked a bit through the book on Marie and did a bit reseach again I found an aspect quite interesting: That Marie already with seven years started to write her thoughts into diaries (she used different exercise books with a "depressing black cover") and did not write in French, but in English and it German, even when she was not without mistakes.
Marie, who never felt well in her environment and was often very unhappy saw a possibility to get a distance from the world with her tyrannic grandmother in which she was "locked", and not less she found a chance to write down every conflict, difficult situation and problem, which she could not discuss in the family (in that kind of society it was never possible) and which caused her sorrow as her child.
Not unlike much later her (much younger

) jewish contemporary

she wrote in her book "In this book I put all the things that can consolate me"...and used nine exclamation marks after that, very unsual (and wrote "thinks" instead of "things", something like a freudian mistake

Marie later forgot about those books, but found them one day, when looking through the things of her father, and let them analyse. She called them exercise books with "stupidness", published them, and found an analytical way to interprete those fears and thoughts in that books, which for a person without any knowledge in psychoanalysis might often seem without connection. She saw more and more certain symbols and their meaning and traced back those stories and phantasies/memories to the so called original scene (sexual scenes). She found the explanation that she was more interested in sexuality as child as she could not live like other children and was taken away by her grandmother from usual child plays and games etc and was f.i. not allowed to see other kids nacked etc.
By the way all this was a quite interesting aspect for me, especially because was already so little at that time, when she began with those writings. There are two examples of her writings in the book. An interesting talent and work of a very young child!