Hi RR,
I've searched transcriptions of Anglesey BMD records for references to your Rowland relatives and found the following. Much of this confirms what you already have but there are some discrepancies.
Children of Edward & Sarah Owen:
Owen Rowland, son of Edward and Sarah Owen, born 19 Nov 1839 - Holyhead
Margaret Jones, dau. of Edward and Sarah Owen, born 13 Dec 1843 - Rhoscolyn
Lewis Jones, son of Edward and Sarah Owen, born 27 Jun 1850 - Rhoscolyn
Edward Rowlands, son of Edward and Sarah Owen, born 11 Nov 1853 - Rhoscolyn
John Rowland, son of Edward and Sarah Owen, born 13 Sep 1856 - Rhoscolyn
(I assume Margaret and Lewis were registered as the children of Edward Jones, he being named Jones patronymically from his father John Rowland).
Deaths:
John Rowland 5 Apr 1868 age 11, at Bont Ry Bont, Rhoscolyn, son of labourer Edward Rowland.
Edward Rowland 6 Dec 1872 age 54, at Bont Ry Bont, Rhoscolyn labourer. Informant Lewis Rowland.
Sarah Rowland 15 Nov 1859 age 44, at Four Mile Bridge, Rhoscolyn, wife of agricultural labourer Edward Rowland. (I believe Bont Ry Bont is Welsh for Four Mile Bridge).
Marriage Lewis Rowland and Margaret Jones:
As you say occurred at Rhoscolyn Parish Church 18 Feb 1873 (Rhoscolyn 1, 59, 117), but Lewis's father is recorded as Rowland Rowland.
There is also a marriage at Llanfair yn Neubwll 21 Nov 1838 which may be the marriage of Sarah Owen and Edward despite Edward being referred to as Edward Williams - which I cannot explain.
The details are:-
Edward Williams (full age, bachelor, labourer) of Penybont, son of John Rowland, labourer married Sarah Owen (full age, spinster) of Cymmuran, dau of Owen Thomas, labourer. Edward and Sarah both signed by mark. Witnesses Owen Thomas (by mark) and David Williams.