Brenda, do you have a source for Mary's mother being Pricilla Cotton Wills, wife of Richard? see Michael Cotten's post # 2449 in the Cotton family Genforum. I've worked with his data, and I cannot find how this Priscilla could be Mary's mother.
There is a will for Apperently a brother of Mary, Thomas R Wells which I found in the Wills family Genforum #1240: will extract from Halifax Co, NC, Will Book Feb Court 1820, p 650. Thomas R. Wells: wife Peggy - Baker or Baber Plantation Sally Alsobrook after mother's death nephew Thomas Godwin Willoby Wells sister Elizabeth Godwin sister Mary Cotton son of Willoby, Thomas Wells grandson Pope A. Alsobrook sister, Olive -?- sister, Zilphey Parker
So the Mary Cotton is apparently our Mary Tart Cotton. And if we play with her suppositional dates...I find her born no later than 1745-48, since her oldest boys by Mr Tart were still minors in John's 1782 will, which means they were born after 1761, and William Cotten has the first one being born in 1764. She was not the oldest Wills child.(At least her brother Thomas listed her sister Elizabeth before her, and we don't have a birth order for Thomas.)
Okay, her mother could have then been married say by 1744-45, which puts her only 15-16 when she started her family.So this could work...but it is tight.