Significance of pendant in Rob Roy (1995)

According to the FAQ entry "Was Cunningham the bastard son of Lord Montrose?"

Many viewers come to that conclusion, and the story strongly suggests (albeit not explicitly) that Archibald Cunningham may, indeed, have been the bastard son of Lord Montrose or, at least, that Archie was somehow related to Montrose.

Some clues are:

(1) When Archie is introduced to the Duke of Argyll, Argyll asks Montrose whether Archie is "another of your likely lads?" (Montrose doesn't reply),

(2) Archie was sent to Montrose by his mother in hopes that the climate might "cool the fever in his blood", evidence that Montrose and Archie's mother know each other,

(3) Archie says that his mother has narrowed his paternity to three candidates the Earl of Rutland, a secretary to the Spanish ambassador, and "some young buck" (perhaps Montrose) who raised her skirts at a masked ball,

(4) Montrose and Archie sit together for a portrait, and

(5) At the end of the movie, Montrose retrieves the necklace bearing a portrait of Archie's mother from around his neck. Some viewers have suggested that, if Archie is not Montrose's son, he might be the son of Montrose's sister or someone else to whom Montrose would agree to favor her son with his assistance. Unfortunately, their relationship cannot be verified, because Archibald Cunningham is a character made up for the movie and has no historical precedents.