You might perhaps have quoted a few dates in your question.
Also Balquhatstone is in Stirlingshire, not in Lanarkshire, so this may not be the most productive place to post this particular question.
Scotland's People lists the marriage of John Inglis to Agnes Salmond in New Monkland in 1739, and seven children of John Inglis and Agnes Salmond, baptised between 1740 and 1761. The eldest was baptised in New Monkland and the rest in Slamannan. This rather suggests to me that Agnes was from New Monkland rather than from Torphichen.
What you have is not George Waddell's will as such, but his marriage contract, drawn up before his marriage to Elizabeth Gaston Ralston, and the reference to Agnes Salmond is to property disponed by her to a George Waddell who was deceased at the time when the contract was drawn up, that is, in 1844.
The then deceased George Waddell is most likely to be George Waddell, 8th of Balquhatstone, who was born in 1739 and died in 1813. His son James inherited but shortly afterwards sold the estate to his distant cousin George Waddell of Ballochney, who later married Elizabeth Gaston Ralston.
So Agnes Salmond or Inglis must have sold the property to George Waddell some time before 1813, possibly in her old age.
I suppose what you really want to find out is whether Agnes Salmond or Inglis was a sister of Patrick Salmond, son of Patrick Salmond and (possibly) Jean Main, who was baptised in Slamannan in 1742? I note that this couple had seven children between 1742 and 1760, the first two baptised in Slamannan, the next two in New Monkland and the last three in Glasgow. The youngest of these was named Rebecca.
Noting that the family of John Inglis and Agnes Salmond were Jean, Margaret, David, Mary, Lilias, Anne and John might suggest that Agnes' mother's name was Jean, but there is quite a gap between Jean and Margaret, so there could be one or two children missing from the baptism records.
I see that Patrick Salmond and Rebecca Marshall had a son John baptised in Torphichen in 1719. The mere fact of them having a child baptised in Torphichen in 1719 does not necessarily mean that they were living there 20 or so years later.
You need to look at the Registers of Sasines for Stirlingshire between 1780 and 1820 or so to find out when Agnes Salmond or Inglis sold the property to George Waddell (1739-1813). This should provide a reference back to earlier sasines that will tell you when and how she acquired the property, and then possibly back to when her father acquired it. It is very likely that one of the sasines referring to Patrick Salmond will tell you who his wife was.
If George Waddell acquired the estate from Agnes Salmond or Inglis before 1780, you would need to consult the earlier sasines. I seem to recall that the earlier ones for Stirlingshire have not been indexed. It would be very good to learn that I am wrong about this, but if I am right it would be a very long job to search them.
The Calendars of Abridgments of Sasines from 1780 onwards have been digitised and can be consulted in the Historical Search Room in General Register House in Edinburgh. If you can't go there in person you will have to hire a professional searcher, as the National Records of Scotland staff will not undertake extensive research for you.
