Through the magic of clicking the wrong button at the wrong time to check out Greig-Duncan, my extensive reply to Teribus has vanished forever.
The gist, however, is that Jack and meself are correct about the subtle difference between "earliest possible" dates, "apparent dates," and actual dates. And Alan is correct that the "Twa" lyrics need not be historically accurate or consistent.
The countries mentioned in the refrain could have been modernized at almost any time during the French wars without being attached to any surviving text. For all we know, the Scottish reviser of the 1706 song was inspired by knowing *only* the modern refrain to its evolved modern tune: he may then have discovered "Over the Hills and Far Away" in a book or on an old broadsheet and decided to rewrite it in the Victorian era, keeping the Napoleonic refrain he knew because it was catchy, prefabricated, and able to impart some Napoleonic glamour. (But that really *is* just conjecture, just like is my sudden notion that he may have been spurred on by reading Kipling's "Barrack-Room Ballads.")
By way of comparison: the anachronistic "Battle of New Orleans" was written well into the twentieth century, not in 1815, and John Tams in the 1990s created a number of "Sharpe" stanzas to "Over the Hills and Far Away" - all of them cleverly designed to resemble relics of the Waterloo era.
Tattie and Jack: the link led me to the U. of Edinburgh, but a click there on the Greig-Duncan link refused to work.