All that said, I actually loved this “twist” and, when I thought about it a little longer, it made complete sense to me. I don’t believe that the “Mummy” chats were ever supposed to be seen as a delusion or an hallucination. Eleanor knew that they weren’t real even though we didn’t - she was never schizophrenic. There are a number of moments/arcs throughout the book that hint at this kind of behaviour from Eleanor, e.g. her chats with Polly the plant (she clearly stated that she knew it was a plant and didn’t talk back), her belief that the relationship with the musician is real and has begun, despite never having met him (she both believes it is real and is fully aware that it is not at the same time. We learn this as we read her private thoughts, as well as in the way she talks about the musician “project” with other characters). 

With this in mind, given that the story is told by Eleanor herself, the implication is that she has always been fully aware that her conversations with Mummy were imagined. An overarching theme of the consequences of her toxic upbringing and relationship with her Mother was that Eleanor was never able to truly become herself. She couldn’t think her own thoughts, speak her own words. Essentially, the line where Mummy’s thoughts/voice ended and Eleanor’s began was blurred to the point of it almost not existing. 

With that in mind, it makes complete sense to me that her chats with Mummy are completely imagined, in her own head - until she chooses to unpack and process her past and she is able to break the trauma bond, discover her own inner voice, and move on.

At first, I was confused by this unexpected turn in the last four pages but, upon reflection, I believe it to be the perfect ending, completely congruent with the storyline and the character, and I would hate for it to have ended differently!