Hi
Yes i think you have made the right connections and Mary's marriage to John Northey was very short-lived. There is definitely a Doidge connection on the Waterfield line as one of Mary and Robert's children was named John Doidge Waterfield.
Also, in 1901 my great grandfather, James Waterfield had a 7 year old child called Rosina Doidge living with them, but I've yet to find out who she was.
Anything you have on Mary Ferris would be appreciated, thank you.