So I think I may have the right family after all and perhaps Edward Smith is Edward Stephens/Edward Stevens but where is he in 1851?
Family Search baptism
Edward: 5th August 1846 Lllananno, Radnorshire - mother Mary Stephens
1891 RG12; Piece: 2285; Folio 78; Page 30
Joseph and Mary with grandson Edward
Stevens b 1874
1881 RG11; Piece: 2865; Folio: 148; Page: 40
The two Edward
Stephensand a young Mary
A possible baptism on FS for Mary Stephens, Ruabon 1831

but possibly not- better wait for marriage certificate
Mry Burton must have been included in that tree because of her birth place

I was put off by the census references you gave but all is well

good luck
heywood