Given that William would most lightly have married between 1821 and the birth of John (Henry) Evan(s) I note ; William Evans married Margaret William at Llanelli 23 Apr 1821 William Evan married Margaret Adams at Llanelli 28 Dec 1821 William Evan married Hannah Evan at Llanelli 10 Dec 1822 William Evans married Margaret Jenkin at Llanelli 27 Sept 1825 William Evans married Sarah Mainwaring at Llanelli 13 Nov 1825 William Evan married Elizabeth David at Llanelli 21 Mar 1828
Thomas Samuel married Margaret Hugh at Llanelli 25 Nov 1828 David Samuel married Margaret Rees at Llanelli 17 Mar 1829
I would suggest that John and Margaret of Heol Hen in 1851 where the couple John Evan and Margaret Harries that had married in the 3rd Qtr of 1850.
Am I correct in assuming that you are working on the premise that John Henry Evan(s) had married Margaret Elizabeth Samuel prior to their emigration and the birth of William. If this is the case, then one must question why their mrge is not listed Civil registration for the U.K. 1845 ~ 1861?