I've attached a link to Pam's thread on tracing living relatives which is more suitable for your last query than using census and bmds.
http://www.rootschat.com/forum/index.php/topic,171449.0.htmlI'm beginning to wonder if Jane 'lost' a few years from her age when she married a younger man, she would not have been the first.
The family I keep coming back to is one in the right area, although their address is not Achddu Ishaf - no Griffiths family there in 1871.
1871 The Village, Llan RG10/5470 folio 17 pg 9
John Griffiths head mar 60 labourer at copper works b. Llanelly
Ann wife 57 b. Llanelly
John son 24 labourer at lead works b. Llanelly
Thomas son 13 scholar b. Pembrey
Mary Anne grand dau 11 scholar b. Pembrey
Edwin (? unclear) grand son 7 b. Pembrey
1861 Bankybrooks, Pembrey RG9/4115 folio 40 pg 48
John Griffiths head mar 51 copper works lab b. Llanelly
Ann wife 49 b. Llanelly
Jane dau 23 b. Llanelly
John son 14 scholar b. Llanelly
William son 10 scholar b. Llanelly
Thomas son 3 b. Pembrey
Mary A dau 5 months b. Pembrey
1851 Towynbach HO107/2469 folio 123 pg 11
John Griffiths head mar 40 copper smelter
Ann wife 38
Jane dau 13
Ann dau 8
John son 4
all born Llanelly
can't spot them in 1841, but with rounded down ages, no place of birth, and no relationships, it's not easy to find common names on the 1841.
Given his occupation of fireman on the marriage cert, does not mean that John would show up as fireman on the census. The enumerators were limited in the descriptions of occupations that they could use. I would think something along the lines of copper smelter is more likely. The 1871 address is the next page from Furnace.
I noticed that the Preston family have a 6 yr old niece, Betsy Griffiths, living with them in 1891. Any idea who she belongs to? It would be easier to find Jane if there was another sibling to look for as well.