OK, so this may sound a little over the top but I've been mulling over the problem of Robin, especially in light of that bizarre ending. My thought was triggered by the Doctor's statement on page 105 that it would be much better if people were born at death's door and gradually grew younger until at last they were infants searching for a womb to crawl into. I suspect, rather than simply going insane, this, or something like it is what is happening to Robin throughout the novel. She, knowingly or unknowingly, is seeking a return to childhood innocence and ultimately the womb, where presumably she will eventually be reborn. (There are several references to reincarnation in the novel--all made by the doctor).
Early on she is described as having a childish face. Later the doctor describes her as someone who needs "permission to live" and someone to tell he she is an innocent (p.125) On page 155 he describes her as an innocent. Nora refers to her as a child and ultimately as her child (p.166) and raises the whole issue of incest again.
I think that may be why throughout the novel, Robin is increasingly driven to wander away from every adult relationship she has. And as the story progresses those wanderings become longer and more intense. Ultimately she wanders away from Jenny and returns to a place where where Nora - her mother- can find her. That scene with the dog reminds me more than a child playing with a dog than anything else.
Does this make sense to anyone else, or am I totally crazy.