Thank you everyone for all the advice and excellent information.
John, you asked about Ross Priory. I think you are correct in thinking it should be Ross in Buchanan. The notion that it was the priory comes from notes left by a great uncle, as follows:
"The McAlpines were proprietors of the Ross Priory (between Balmaha and Balloch on Loch Lomond) for 700 years. Then they sold it to the Duke of Montrose. One of the sons – Daniel McAlpine – took the farm of Culliemare near Rowardennan. His daughter Agnes [sic, should be Jean] married John Graham of Frenich near Aberfoyle. Their elder daughter married Andrew McFarlane of Glengyle, Cashell and Craigievairn..."
All of the relevant McAlpine deeds and other documents catalogued on National Records of Scotland site relate to Ross, Calziemore and Lurg. So a fanciful addition of the word priory at some point in time...
The Andrew McFarlane and Jean Graham (daughter of John of Calziemuck who died at Frenich in 1803) in these notes are the Mr and Mrs McFarlane visited by the Wordsworths and Coleridge in 1803.
I think a good contender for the McAlpine relative mistaken for a brother is James McAlpine (born 1755) or one of his sons. All of Jean Graham's brothers had died by 1803. Captain William McAlpine's will of 1809 describes his brother James as residing at Cashell (Andrew McFarlane's farm after Glengyle).
As to James McAlpine's mother, I think she may well be Ann McFarlane, Daniel's second wife, and not Agnes McKerchar under an alias. Two reasons: the witnesses to James's baptism are Andrew and Donald McFarlane of Salachy. Plus his birth is seven years after the last recorded baptism of a child of Agnes McKerchar.
According to the same great uncle, my McFarlane line were in Sallochy for generations. "Glengyle" Andrew McFarlane was born in Sallochy in 1770. His father was Peter/Patrick and grandfather another Andrew (possibly the one who witnessed James's baptism).
Catherine