Evan Roberts married Mary Morgan on 23 Apr 1835 in Llanbadarnfawr, *Cardiganshire* (where they are still living on the 1851 census).
Llanbadarnfawr, Radnorshire is a completely different place!
But which Llanrhaeadr in Denbighshire?? Llanrhaeadr (yng Nghinmeirch), to the south of Denbigh, or Llanrhaeadr (ym Mochnant), to the west of Oswestry? Gut-reaction would suggest the first?
A copy of the marriage certificate from the second marriage should give you Evan's father's name.