Apologies for such a late reply to your questions. I hope you have had some joy & that you are still exploring the family!
I have no extra information at all to add, but have been doing some reasearch & am also trying to find my partner's grandmother. From word of mouth, her family were from Cerrig Cynrig. Her name was Eleanor Griffiths and she was born circ 1891. I am in the process of trying to find her marriage certificate so as to try to get some more clues, but I am wondering if maybe she is a relation (grandaughter) to the son Griffith that you are trying to locate, and that his first name had been used as the family surname. Hope you don't mind me adding this comment - we have no other clues that help us try to find her on any census -1891/1901/1911.
A bit of a random ask but worth a try!