I believe the Joan KELLOW who married John FRENCH, was, indeed, very probably the Joan KELLOW bap 13 Jan 1767, Tintagel, Cornwall, daughter of William KELLOW and Catherine BISHOP.I have Joan FRENCH's burial record as 31 Dec 1836, Tintagel, Cornwall - this states that she died aged 68, which is only a year adrift.
I have nine children for Joan KELLOW and John FRENCH, including a James FRENCH, bap 20 Dec 1803, Tintagel, Cornwall.However, I don't have a middle name "Richard" for him, though I suppose this could have been added later.
As your James Richard FRENCH died before the 1851 census (which, if he was in England, would have helpfully shown his place of birth), how do you know he was from Tintagel?(Just checking! ).
Do let me know if you think I may be able to help/hinder further!