I'd assume that N. N. Northcross would be Northern (or Northen) Nelms Northcross born 1809 -- A slave owner -- or a descendant named after him.
I'm most curious about the information from your new genealogist.You say: "a recently-unearthed (to me anyway) Penny cousin [regarding a tie to the Northcross family] tells me that her gggrandmother 'was Mary Northcross, daughter of John McGlamory Northcross and Sarah Larrison Oakes. John was the son of Thomas Northcross and Hester McGlamory. Thomas was the son of Richard Northcross who died in Trenton, Tennessee in 1839.' Surely this is the same family as the one you are researching, and I hope that I have provided a lead or two in repeating this information."
Most of the information I've seen assumes that Thomas Northcross' parents were either -- Richard Northcross (1720 - 1792) and Jane Stratton (1725/35 - 1807) or Jane Stratton and a Joshua (or Joseph) Northcross.
This tidbit demands a different parentage for Thomas. This says that his father was named Richard. And that this particular Richard died in 1839 in Trenton, TN. Very interesting. Thanks for passing this on.