Hello,
The Catherine does seem like a possible link, especially with both mentioning’s of Eliza & Thomas, and thomas with the correct birth date,
With John Dying and Catherine remarrying this could also make sense on why Thomas Jones may have appeared on the 1871 census with a married Mary Ann in Pontnewydd, whilst Catherine remarried and went elsewhere,
I did find a possible baptism
Mary Ann Jones
10 May 1835
Llangattock Lingoed
Parents Elizabeth & John
Father Labourer
Their was another with Mother’s name Eleanor however i seem to have lost that record,
These are potential candidates aswell, as we know census ages seem to be unpredictable
Only concrete information we do have is the Eliza Jones and Thomas Jones, and Llangattock Lingoed.