I would like to hear from a doctor about people with her disability and her own level of disability. It seems her condition would not worsen: "HPE is not a condition in which the brain deteriorates over time."I have always wondered who looked after the children if both parents were away for work. A nanny’s testimony would be useful regarding Nora’s abilities, although the school principal confirmed the parents’ testimony. But the more information the better, I suppose.
Holoprosencephaly - Wikipedia. It also seems her case was exceptionally mild, more severe cases have severe disfigurement and don't survive more than a few years.
My problem with the statements that Nora could not possible have gone off on her own, is they don't take into account that, in spite of her disability, she was a teenager, growing and changing fast. At some point, every parent discovers with shock that their little girl is determined to do things they've never seen them do before, and has emotions and desires that they never expected. It seems to me a possible case of over-protectiveness, that her parents believed that she would never, could never, grow up.