This probably won't be a direct help to you in your search, but it may help somehow. With so many Roberts families in the area, it's difficult to pin down the right one!
My G G Grandfather was Evan Roberts (b 1815 Llanfor) who married Jane Davies from Ty Isaf, Melin-y-Wig, Gwyddelwern in 1838. On their marriage certificate it gives his father as John Roberts, with one of the witnesses as Robert Roberts. I assume Robert R could be a brother. Evan and Jane later had 8 children, Elizabeth, John, Margaret, Humphrey, Jane, Evan, Mary and Robert, all except Elizabeth born in Llanfor.
When Evan R Sr died in 1866, the informant was Caldwalader Roberts, who could have been a neighbour or another relative. I haven't been able to prove any more than this.
I haven't found a David in the family, but who knows?
Good luck.