I've done a welsh newspaper search for Penrallt Eifed which has brought up the report of the death and burial of Evan Evans aged 21, reported on 4 July 1884. He drowned in the River Teifi on June 18. He was buried in Capel y Wig (suggestion is this was the family burial location) and mentions he was a full member of the Capel Isaf independent church in Llechryd, where his father was a deacon. sadly no other helpful family background
The 1881 census confirms Penrallt Eifed as William and Margaret's family home, but also suggests both Evan and Mary Ann were born in Llangrannog. I note you've said "born in..." from their marriage cert, but in the UK this records their parish of residence not place of birth, so looking for Mary born in the LLangoedmore area would be a mistake, I think..
Llangrannog comes under Newcastle in Emlyn - unfortunately there are quite a few Mary Evanses and Evan Evanses born in that period, and the new GRO indexes have quite a few with mother's maiden name Jones