Hi Barb - from the Tower of Sark inscription, it mentions Anne Harkness who died in 1807 aged 45.
This Anne Harkness was therefore born about 1762, if my maths is correct!
From the same MI Anne Currie, wife of George Harkness, was born about 1702, and would have been 60 when this Anne Harkness was born.
Therefore I think the MI refers to an Anne Harkness, daughter of James Harkness and Jane Irving, and probably granddaughter of George Harkness and Anne Currie. The wording isn't clear but that's how I would have read it.
So the Tower of Sark MI doesn't actually negate the theory that Anne Harkness, wife of Thomas Johnston, was daughter of George and Anne. (If they definitely had a daughter Anne!)- the Anne on the gravestone at Tower of Sark was probably a granddaughter of theirs, not a daughter. There is a link with Goldieslee in the two families, as well.
Your theory sounds more likely, but possibly you shouldn't rule out any possiblities yet! There are a lot of Harknesses about.
cheers
robbiesmum