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Just for the record: there have been some mentions of and research on this particular John Davies on Casebook and JTR Forums before, though not very extensively. From a summery of the inquest account (https://newspapers.library.wales/view/3387680/3387682/16/) published in the Cardiff Times on 10th September 1881 we find out John resided at 18 Lewis Street, Dowlais, the 1881 census record of which indeed lists John living with his wife Margaret (23) and two children as well as an additional pair of lodgers from Ireland.
Although I was unable to find a marriage record, either in Carmarthenshire (where John was born) or Dowlais (where they resided in 1881), I think I found Margaret (John Davies' widow, born Dowlais) in the 1891 census, living on 8 Spring Street in Dowlais as widowed daughter of Mary Herbert (also widow).
In the 1891 Dowlais census Margaret is exactly 10 years older than in the 1881 census, as is her son David Davies, who was 5 months in the 1881 census (now listed as 10 years old in the 1891 one). Margarets maiden name , we may conclude, was Herbert and she was alive and well in 1891, so I'm afraid this John Davies and his poor widow are yet another blind alley in our search for Mary Jane Kelly.