Sorry, but I am now completely confused.
I have OPR's for Betty, bpt August 5 1820 at Hawick and her MI suggests born 1761.
Baptised in a different county aged 59?
Is this the same Betty
the MI records Betty's death as 20 April 1864, age 83
and if so who is she?
Have you looked at her death certificate?
If she was 83 in 1864, that implies that she was born in 1780/1781, not 1761 (unless you are talking about two different Bettys).
Why do you think the Betty Harkness baptised in Hawick in 1820 is a daughter of
your Thomas Harkness? You said that Hannah died in 1827, and Thomas in 1832, and they are both on the same stone. So Thomas cannot have been widowed and remarried. Now, if Betty was born in 1781, her parents cannot have been born much after 1765, and more likely around 1755. So by 1820 Hannah would have been
at least 55 years old, therefore she cannot be the mother of the Betty Harkness born in 1820.
May I suggest that you post a full transcription of the MI, rather than just extracts from it? It would be much easier to understand it.