Can I check ages? You mention "abt 1779" for David and "1775" for Mary. If I have the right Censuses -living Brynffynon '41 with Evan and Brynyfunon '51 with Rees?
1851 - both aged 75, born c. 1776
1841 - both aged 65 (born 1772-1776 allowing for rounding down)
Can you confirm if they can be found in '41 and '51 and, if so, confirm census reference nos?