Hello again osprey,
Some brilliant detective work!
Yes, you are looking at the right family in 1841,they were living in Sarnginini Fach.
Well done for noticing where they were born,it had slipped past me,curious now as to where Mary and David were born-Llandyssul is a possibility as it straddles the border between Carms. and Cards..
1851 next,looks like paydirt again,as most things tally.We also have the appearance of Ann[b.1844,?] and James,10 months,who looks likely to be Mary's child as you say.By now David,Sarah and Samuel have disappeared.
1861 again looks good.Mary has now disappeared but her son remains.I can account for Samuel in this year as he appears in RG/9/4189 folio20 page1 with wife Elizabeth and son Vernon.
I can account for Samuel and family in 1871-RG10/5552 folio8 page9 and by now his family have increased to include John,Rachel and Margaret as well as Vernon.
By 1881 though Samuel,Elizabeth,Vernon and John have all disappeared leaving Rachel and Margaret as well as 3 more children-Sarah,David and Hannah as Boarders in Elizabeth Rees' school[RG11/5439 folio19 page7.And here lies the mystery.John I can maybe account for but the other 3 are a mystery.
I can also account for Rachel,Margaret[my G.Grandmother],Sarah,David and Hannah.
As you can see some missing parts to the jigsaw.I would be grateful if you could shed some light on these,but these would be a bonus you've already filled quite a few holes.The marriage and death regs. look good also,I shall go ahead and order them.
Thank you,Diolch yn fawr,
David