According to G F Black's
The Surnames of Scotland M(a)cKean is a form of MacIain, which simply means 'son of John'. Black also says that when the MacIains (MacDonalds) were driven out of Ardnamurchan in the early 17th century some settled on the east coast. (I know that Larbert isn't exactly on the coast, but it's certainly well east of Ardnamurchan.)
So it's perfectly possible that your McKeans are descended from this earlier migration.
I think your chances of proving any connection to either the massacre or the Ardnamurchan MacIains are very small, because it's highly unlikely that enough records have survived to prove anything.
A lot of people in Larbert worked at the Carron Iron Company, which was founded in 1759. See
https://en.wikipedia.org/wiki/Carron_Company and it would not surprise me if your Thomas was one of those who moved there to take up employment. There are archives of the Carron Company, including records of employees. Some of the company records are in the National Archives of Scotland and some in the National Library of Scotland. Go to
http://www.scan.org.uk/ and search the catalogue for 'Carron Company'.