Hi Catherine,
I don't blame you a bit for being confused, especially when it comes to the two Hughs. That's a good question regarding why we assume this Jean Wagstaff is Jean M.S. Brown.
Here are some thoughts...
Jean's husband Joseph Wagstaff died 1833, so we know that Jean would have been a widow in 1841.
All of the other Wagstaffs in Newmilns in the 1841 c. are Jean and Joseph's children and their families (no indication of a second Wagstaff family in the area).
As you say, the age fits. There are no other known female Wagstaffs that would be this age in Newmilns, except for Jean the wife of Joseph. It's possible, of course, that this is another Jean Wagstaff that we don't know about.
We have this OPR burial record for 19 Feb 1847 (from Claude Wrathall's transcriptions): "old Weddaw Wagstaff Tounhead Newmilns She is intered in the northmost Graff of George Smith Lair in the Brouns new Ground at the west dyke."
(note she was buried in the "Brouns new Ground")
These burial records go back to 1811, and we have no other burial record that indicates the death of Jean.
There are no other Jean Wagstaffs (except for her descendants) that appear in the census after 1841.
So, I guess our assumption is based on the age of Jean Wagstaff in the census, and circumstantial evidence. This certainly could be flawed, but based on what we know I feel pretty good about assuming this is our Jean Brown.
Best,
Trish