"A detailed ballad about say Battle of Sheriffmuir is unlikely to have been about an earlier battle"
You really can't base anything on these assumptions Steve.

"The ballad of 'The Haughs of Cromdale' is taken from Hogg's Jacobite Relics. It was originally produced to describe this battle and as it has catchy tune was soon being sung all over the Highlands. This seems to have been too much for some unknown bard and, in an effort to redeem the description of this defeat of the clans in 1690, he added on a somewhat high-flown description of Montrose's victory at Auldearn over the Covenanter army in 1645. Thus the two battles, forty-five years and a considerable number of miles apart, were unceremoniously joined together. The gallant Montrose, who had been dead for over forty years, was brought to life in verse to win another battle. The result is horribly muddled ballad, but one which has been immensely popular. To the strains of the pipes playing this tune the Highlanders have charged and won battles all round the world."

That appears to be how ballad making worked.
Jim Carroll