You are not alone...
Mary Robbins exists. See her father's profile for the information on her that comes from Anderson's Great Migration Series. That's it. Obviously he knows nothing about Nicholas Robbins. Or another husband that he wants to commit to.
Torrey's New England Marriages knows nothing about a Nicholas Robbins marrying anyone, let alone Mary Hinckley. The only Mary Hinckley mentioned is a questionable marriage to John Weyburn about 1671.
The son John Robbins, got his own book. The Robbins Family of Cape Cod (1917), only it clearly states no record of birth or death.
It looks as though Nathaniel may be total internet fiction.
When did Mary Hinckley die? Without a positive ID on her husband, we will never know. After the date of her father's will is about as close as we're going to get.
I would spend a little more time checking for parents of John Robbins, then I think the best course to take is disconnect child John from both parents (with appropriate sourcing on his profile). Disconnect Nathaniel and Mary Hinckley, mentioning on her profile that some ancestry trees have this relationship. Then merge away Nathaniel.