I'm afraid Henry Newton Guess has no connection to Thomas K. Guess.H,N. Guess was a member of the Henry Guess family, who were not related to Thomas K. Guess at all. H.N. Guess was born 1851 and died sometime after 1900.H.N. Guess never married and left no children.
Regarding the parents of Thomas K. Guess:It is generally accepted through family recollections that Squire Guess was the father of Thomas K. Guess.However, as you have found, there are no written records to prove the connection.William Marion Guess, Jr. does not have the answer either.Both William M. Guess, Jr. and his father William M. Guess, Sr., who died in 1987, have been interviewed and questioned endlessly by various researchers, and they simply do not have any information about the family further back than Thomas K. Guess.William M. Guess, Sr. put together his family tree around 1970, and it goes back only as far as Thomas K. Guess. I have a copy of his hand drawn family tree I could send you.
I would think if there is a record to be found that connects Thomas K. Guess to Squire Guess or William Guest, Jr. it would be in SC, where Thomas K. Guess was born and where both Squire Guess and William Guest, Jr. lived out their lives.Thomas K. Guess was an adult when he moved to GA, and he was apparently the only member of his family to make that move.
There is DNA proof of sorts that shows William Marion Guess, Jr. is closely related to others thought to be descendants of William, Jr., through John Guest, a brother of Squire Guest.This is hardly the type proof required for the DAR, but it at least confirms the presumed family connection to William Guest, Jr. is indeed correct.The problem of finding written proof remains.