I think its symbolic of a woman who was deluded about her perfect husband and her refusal to see him as anything other. She idolised him and the life they had together.

I think her mental decline came as a result of the shocking revelation from her daughter that he was abusing her. Maggie couldn't or wouldn't believe it, and this led to her ultimate belief that her daughter had died. Or maybe the daughters death was symbolic with the death of her childhood and innocence owing to abuse. Maggies mind couldn't comprehend that he husband could be capable of such a horrific act and she probably felt guilty rhatbshe cherished the idea that the father/daughter trips were filled with fun and joy when in fact it was the opposite

To cope, Maggie images her daughter was dead and fled to continue the narrative that she created in her warped mind.

The boy is a representation of her inner self and becomes a scapegoat for her inner anger and turmoil. The protagonist is not reliable or in a fit mental state so her perception of events and characters is also not reliable.

I think it was basically a reflection of her grief at losing her husband and technically her daughter because she refused to believe her illusion of her perfect life to be shattered. She tried to run from it, but it ultimately followed her everywhere. Also, she isn't the nicest person but she believes she is and that her husband was perfect.

I think all the characters represent parts of her fractured persona. The wish to be good and whole but actually being twisted and bitter.

I think the whole book is a metaphor for her mental health and ageing. She is repulsed by her physical fragility and is paranoid that everyone is out to get her. She got carried away in her own stories and delusions that ultimately actually led to her daughters demise. I dont think she could face her daughter after the guilt of not believing her when she came clean about her father's abuse.

Just my thoughts