Hi Kay.I just heard from another researcher who told me that the "History of Bridgewater, MA" says that the Nehemiah ALLEN born in 1715 was from the Samuel, Samuel, Nehemiah ALLEN line, he being the second Nehemiah.Could the first Nehemiah (here) and your Nathaniel have been brothers?
Also, during my on-line searching, I came across mention of a Nehemiah ALLEN, b~1756 MA, marrying a Mary ALLEN, b1760 Middleboro, MA.I don't see a date of marriage.But, this record shows that Mary was the daughter of Nehemiah and Abiah (THOMAS) ALLEN .....
I found the record atwww.familysearch.org
But, going back to what you said, your Mary ALLEN b1760, daughter of Nehemiah, married George LEONARD.Is it possible she married twice?
I didn't see where the Nehemiah b1756 was living, or what his line was.