Can you find any more about the Farcet girl, in order to rule her out, or to add circumstantial evidence that she might be the right one?
Eg - can you find a marriage for her, find her on later censuses? Who are her siblings - do any tie up with the witnesses at your Mary's marriage, or turn up living with your Mary on a later census etc etc.
I haven't checked with a map, but my memories of that area were that Whittlesey and Farcet were very close.