Nah, Robyn, you're on the ball- but Catharine as Thomas daughter has come up before- I think we've all thought that would be a very tidy solution.
The 1814 date for the obstacle and the hpothesis that Thomas received a letter from Elizabeth Ransom (Durrell?) at that time could add weight to the theory. The letter presumably being advice that daughter Catharine was on way, either as free woman or as convict. If Elizabeth, the original Mrs Ransom, was on the way, then she must have died on voyage, never left, died on arrival or met someone else, otherwise she would have appeared in Hobart as Mrs Ransom, and none of this thread would have been needed.
If Catharine was Thomas 'daughter, she was either conceived during a nuptial visit by Elizabeth to the gaol/hulk just before Thomas left in 1788/89, to be born in 1790/91 , "Hi Thomas, long time no see, Guess what", or Catharine was born before 1787 .
If she was his daughter, she was Catharine Ransom, not Catharine McNally, unless she had married a McNally, in which case was was the widow mcNally.
If she was his daughter, she would have been acknowledged as such, as Mrs McNally or Miss Ransom, and everybody would have known she was Thomas' daughter.
She would have been named as Thomas's daughter in his will, not as his friend.
I think that there are too many strikes to run with it- but it sure is a tempting idea!
David