Owen comes from the MIs of Lledrod with wife Jane.I think you have inferred that her maiden name was Richards in order to make the "nephew" relationship fit,because the grave is next to that of Joseph,who is buried alone.There is no indication on his MI that he is related and he is given as of Ynys,as are one of the Lewis group.You might well be right in your inference or assumption,but,equally there could be other explanations(intermediate generation for example)
I hae also now found that there must have been a marriage between a Jane Richards of Garn Lwyd and James Davies of Hendre Philip.Their granddaughter,Emily Davies,via her father John(born 1795),appears to have been an early pioneer in women's rights campaigning.
I had seen the will you mentioned,and it again demonstrates the difficulty with patronymics.
I still feel certain that positively identifying the parentage of the nephew Joseph Davies is key to confirming the branch of the Richards family that equates with "David of Lledrod",the father of Edward and,therefore John and James.I feel we have him surrounded,but not yet caught.
Regards
Roger