...one possible new fact being a marriage on 11th December 1818 in Rhoscolyn between a John Rowlands and a Margret Williams, this would tie in with the birth of Edward Rowlands in 1919.
Looking at the baptismal records for Rhoscolyn shows the following who seem to be Edward Rowland(s) and his siblings, which helps link this family to the family at Lletty bach in 1841.
No. 110 - 20 Feb 1819 Edward son of John Rowland, Margaret Williams, Bridge, labourer.
No. 173 - 15 Dec 1822 Mary dau. of John & Margaret Rowlands, Bridge, labourer.
No. 201 - 1 Aug 1824 William [Rowlands] son of John & Margaret Rowlands, Bridge, labourer.
No. 236 - 11 Sep 1826 Elizabeth [Rowlands] dau. of John & Margaret Rowland, Lle??i (unclear), labourer.
No. 263 - 22 Feb 1829 John son of John & Margaret Rowlands, Lletty, labourer.
No. 374 - 1 Jan 1838 Owen son of John & Margaret Rowlands, Lletty, labourer.
Owen's birth was registered - 3 Dec 1837, Owen son of John Rowland & Margaret Williams, Rhoscolyn.
I cannot find Robert Rowland ca. 1835 nor David ca. 1840 but David's death was registered:-
David - 31 Jan 1843, age 2 at Lletty Bach, Rhoscolyn, son of labourer John Rowland. Informant Margaret Rowland, Lletty Bach, Rhoscolyn. The cause of death was whooping cough.
I have checked the birth registrations for the children of Lewis Rowland and Margaret Jones, most confirm the known facts as given by richardemlyn:
Mary, 17 Mar 1877, dau of Lewis Rowlands & Margaret Jones - Rhoscolyn
John, 16 Oct 1880, son of Lewis Rowlands & Margaret Jones - Llanynghenedl
Ellen, 27 Feb 1883, dau of Lewis Rowlands & Margaret Jones - Llanynghenedl
Richard, 12/13 May 1885, son of Lewis Rowlands & Margaret Jones - Llanynghenedl (for some reason the birth was registered twice with different dob - Llanddeusant 14, 31, 151 and 14, 32, 157).
Margaret, 11 May 1888, dau of Lewis Rowlands & Margaret Jones - Llanynghenedl
Edward, 27 Apr 1891, son of Lewis Rowlands & Margaret Jones - Llanynghenedl
Anne Jane, 3 Feb 1894, dau of Lewis Rowlands & Margaret Jones - Llanynghenedl
Lewis and Sarah are different. It seems Lewis was born before the marriage of his parents on 18 Feb 1873. His birth was registered as:
Lewis, 29 Dec 1872, son of Margaret Jones - Rhoscolyn. The father's name was not recorded.
The only possible registration for Sarah's birth I have found is:
Sarah, 21 Feb 1875, dau of Lewis Rowlands & Margaret Edwards - Rhoscolyn
I can find the following death registrations which I think are relevant:
Lewis Rowlands - 8 Nov 1900, age 50 - in a field at Penybont, Llanynghenedl, farm labourer. Informant Lewis Rowlands, son, Penygraig, Llanfachraeth.
Margaret Rowlands - 18 Mar 1942, age 91 - Fron Dirion, Four Mile Brdge, Rhoscolyn, widow of farm labourer Lewis Rowlands. Informant R. Rowlands, son.
Margaret Rowlands - 16 Jun 1905, age 17 - Penybont bach, Four Mile Bridge, Llanynghenedl, domestic servant. Informant John Rowlands, brother, same address.
Ellen Rowlands - 16 May 1905, age 22 - Penybont bach, Four Mile Bridge, Llanynghenedl, domestic servant. Informant Lewis Rowlands, brother, Penygraig, Llanfachraeth.
Lewis Rowlands - 10 Feb 1907, age 34 - Jericho, Llanfachraeth, general, labourer. Informant Richard Lewis, brother, Bontlwyd, Llanfachraeth.
John Rowlands - 17 Sep 1914, age 33 - Penybont bach, Four Mile Bridge, Llanynghenedl, farm labourer. Informant Owen Jones, brother-in-law, Caera Cottage, Llanfachraeth.
Edward Rowlands - 27 Apr 1891, age given as 19 hours - Pantbontbach, Llanynghenedl, son of farm labourer Lewis Rowlands. Informant father.
I'm unable to explain the unexpected names found in some of these records - Rowland Rowland instead of Edward Rowland (1873 marriage), Edward Williams instead of Edward Rowland (1838 marriage), Margaret Edwards instead of Margaret Jones (1875 birth) and Jones instead of Rowland (births 1843 and 1850) - but I do not think they are necessarily the wrong records for these events. Some of the factors involved may have been - illiteracy of the participants, patronymic naming, simple misunderstandings/errors or errors in transcription.
Regards