You’re assuming that persons accusing others are lying, and just picking people to blame to avert blame from Thomas Jefferson. I think they observed conduct in those persons — who shared a family resemblance — that aroused their suspicion.
I have 3 siblings. We have some things alike, like the Cherokee slanted eyes of our grandmother.
Other than that, we are all extremely different. Brother is well over 6 feet tall. One sister is barely over five feet.
Two had straight hair, one of us has very curly hair, one had thick wavy hair. The hair colors vary, two born with brown that stayed brown. Two born blond that later turned brown in adulthood.
Our teeth are also very different. Two needed braces, two did not.
We have all long had children. The brother’s kids look like him and his wife. Even though he has sisters, his kids don’t look like his sisters.
My kids look nothing like my brother. My sisters daughters do not look like my brother.
So my own real life experience is telling me that Thomas Jefferson’s child by Sally Heming, who looked enough like him to be mistaken for him, was not fathered by a nephew of his.
The resemblance would not be very strong at all to Thomas Jefferson and his Carr nephews.