I'm not sure it helps very much,but there is a will of 1807 for a Thomas John of Lochtyn who looks like he might be the husband of Elinor Jones of the 1813 will.He mentions his wife Eleanor(sic) and all the same children(Sarah,Margaret,David) but,strangely,not John.
As far as I can tell he was Mayor in 1794,and may have had some involvement in Penbryn Chapel(money left to him in a will for the chapel).
The daughter Margaret is named in Elinor's wil as the wife of a James Timothy and her burial record suggests a birth of 1774 as a marker for the other children.There is a Sarah Jones,given born 1781("rounded") at Lochtyn in the 1841 census.
I haven't managed to trace David or,importantly,John yet,nor the marriage of the parents
Regards
Roger