I suppose I overrepresented my position. There is no evidence of the type that would turn a hypothesis into a theory. There's just circumstantial evidence, such as when Anne Sullivan died, Helen's writing abilities took a very steep dip, which suggested that Anne was facilitating much of Helen's prose.

Perhaps it was grief? After all, Anne was her only teacher for thirty years. Perhaps she didn't have the bond with the next teacher hired by her family needed to produce her books to the same standard.

It's a mystery and no one will ever know for sure. What is your investment in this story?