Hi Jaju, just thought about the John who died in WW1 - possible his army record is on A******y? Not sure if it was just those who survived had their records published, but that could give you a connection and further details.
Witnesses - Cookesley is a name that married into my family (but she was from London). There were definitely a few Cookesley's living in the Maesteg area at the time, but I forget where I saw the names now

This was 1890, so I have had a look at the 1881 census, and there may be a match or two for your witness:
RG11 5327 / 40 Page 9 - William Cooksly - age 18. Living in Alma Road (which is Maesteg, not Cwmdu, but that's how the reg district has it)