Ellen and Helen are often used interchangedly. My great aunt was Ellen, called Helen, my grandfather's favorite sister who died young. My mother was named Helen and so is my daughter.
I am still having trouble determining your exact question...are you trying to prove that Polly and Mary are one and the same people? If so, what specific information is known about Polly that can connect to Mary. IN the old New England records, Polly and Molly were often used for Mary. More then that I cannot tell you. So I guess you are looking for documentation that ties Polly to Mary?
Or am I off on the wrong path once again? Or are you looking for the descendents of Mary Hoole? Or both?