Yes that burial records fits well as Llaeth du is found in census and on baptism of hannah, pen y lon is also mentioned on baptisms of griffith and elinor.
Did you find copy of burial at national archives?
RE will, i have found no records for Jane and Rowland in baptism or census.
bethesda methodist i have found hugh 1815, samuel 1817, griffith 1820 and hannah 1823.
other amlwch i have found eliz 1806 and elinor 1826 but not found william that you found 1810.
i have robert age 35 on 1841 c miller married to elizabeth ?? with children ann 15, amelia 8 and jane 6 but cant find any future census records for any of them.
Hugh is not mentioned in the will so maybe he died before will was written in 1847.