Data and speculation on people and property as follows:-
Frederick-What in particular makes you think the one servant in 1871(as there are many through the years) is connected to his mother being a Sarah Hughes.I have no evidence for who his mother was.It does seem strange that Jemima,sole heir in her mother Susannah's will is described in effect as Frederick's sister on the 1851 census yet there is no provision in the will for him(or Thomas who may have died?).Perhaps she thought his occupation as butcher was enough for him...or perhaps he wasn't her son.There is a record of a marriage of John Jones to Sarah Hughes at Halkyn in 1811 which looks interesting...perhaps Sarah and Susannah are synonymous.
Frederick marries Sarah Parry in 1853 and they have 6 or 7 children I can find on the censuses.He dies sometime in the 1890s though I need to revisit that as there appears to be a conflict with his appearance in 1895(below).
Frederick can be seen in simple list form in the Slaters Trade Directories of 1868,1880 and 1895(?see later) at
www.historicaldirectories.orgI have not found an accountof his son's death which I thought might be in the North Wales Chronicle.There are options-for another time.
John-He is at RG09 4295 14 9 in 1861 which reveals a link with Jane from the will(later).He marries Arabella Parry in 1865 ,a widow with 2 children from her marriage to Robert Parry in 1854 hence the names in 1871.He dies in 1881.
Jane-I believe she marries Joseph Hill and they have a daughter Susannah who is the niece with John in 1861 and also a son James.I have census refs as necessary.
Jemima-I'm a little uncertain on her fate.She possibly dies in the late 1850s as she is not at the address in 1861 or 1871.However she might correspond as the wife of John Evans who has the same Ochr y Marian address as an Evans family in 1881.If so she marries him in 1859 and has at least one child and outlives him being at RG12 4626 35 6 in 1891.
(The other Evans family have a son David Delta(!) Evans who has a biography at
www.llgc.org.uk)
I cannot currently trace Anne or Thomas
Property
There is an advert(North Wales Chronicle) for the sale of the property by John Jones in 1865,the year of his marriage.It is no longer separately identified after 1881 as per your subsuming theory.However the site is now occupied by an impressive new building and retains the original name.I have a photograph(of sorts) with the permission of the occupier and will send it if you are willing to supply an e-mail adress by PM.I don't believe it proper to publish it here for general viewing and it cannot be attached to a PM.
Perhaps I could help your cousin when the time comes as I'm obviously not too far away.
I have not included too many census refs but I have plenty if you don't!
Regards
Roger