Neno,
I cannot say for certain that there were no coal mines in north Cardiganshire but hopefully the following will help to clarify the apparent anomalies of finding colliers & hewers, and more particularly their wives & widows, in the local census.
If you follow this link
https://lra.le.ac.uk/handle/2381/10235 it should take you to a University of Leicester page in connection with Dr Kathryn Cooper’s thesis on the subject of Cardiganshire’s Rural Exodus. The mention of significant emigration to Glamorgan & Carmarthenshire is telling to me as, by implication, this alludes to the coalfields and allied industries.
However what really underlines the validity of this view is the following sentence* from a wonderful autobiography (*in the context of a young man contemplating employment options upon leaving school) wherein the author writes: “In time a boy might join his migrant father as a collier in South Wales”
The relevance of this is that the author was Tom Macdonald who, as this link shows
https://en.wikipedia.org/wiki/Tom_Macdonald_(writer) was born in Llanfihangel Gennau’r Glyn. Additionally in the 1911 census his family was living in Pen y Garn, Tyr y Mynach.
His autobiography, Y Tincer Tlawd/The White Lanes of Summer, chronicles his upbringing in the area. It is a truly beautiful work and I highly recommend that it should be read by anyone who has a deep interest in the genealogy of rural Wales in the early 20th century.
With regard to the names of your in-laws, it may be that one or more of them (or probably their ancestors) is named among the many characters that Tom Macdonald so vividly brings to life with his writing.