I looked at the 1911 census to see if it was feasible to do a place search. One of the two enumeration districts has 497 pages alone. It would be a mammoth task to search that one by itself.
Margaret Pritchard was born in Llanbeblig, Caernarfon. The family are living in Llanbeblig in the 1891 census. Margaret is shown as Maggie. There is a possible marriage for Margaret in Conwy Registration District in the March quarter of 1907. I cannot find her in the 1911 census, so presumably she left Llangystennin before then.
I think the best way to trace the residents of Llangystennin Hall would be to look at the Trade, or Commercial directories which list the streets, houses and occupiers.
The name of Boyd given to her son, Charles, may be a clue to his father's name.
Helig