Hi,
Based on Montgomeryshire Genealogical Society's Llanwyddyn Parish Registers 1736-1812 (MR/PR/51) I agree with your references for the marriage of Morgan Morris and Catherine Hugh and the baptism of Sara(h).
From the same source: A Catherine Hugh, daughter of Thomas and Elizabeth Hugh, was baptised in Llanwyddyn on 25th April 1765. Though 19 months after this Catherine Hugh and Morgan could have married, the marriage of another Catherine Hugh, also 'of this parish' is recorded.
This is a possible Christening for Edward and maybe his mothers line can be traced to find the Mary Connection:
You're right the line may be through Edward, I'd assumed the connection was via Sara(h). I'd given up on finding much more on Edward if he's the link as in the 1851 Census of Llanfechain Sara(h)'s husband Edward gives the parish of his birth as Llansilin and in the 6th June 1841 census his age is given as 50 which, considering the rounding down to an age of *0 or *5 required, puts his year of birth in the range 7th June 1786 - 6th June 1791. From the Llansilin Parish Registers possible contenders are:
28th December 1787 Edward Jones, son to Robert and Margt. of Sychart [sic]
7th March 1790, Edward Jones, son to Richard and Catharine [sic] of Cynynion
20th January 1794 Edward Jones, son to Edward and Mary of Sychart
The 1841 and 1851 ages give ranges for Edward's date of birth that overlap by 3 months or so meaning, assuming no error in the figures, Edward was born between 31st March 1791 and 6th June 1791. However none of the three suggested possibilities were born in that period.