I've known Iain for about 35 years, and have heard many of his stories which I won't repeat! I am quite sure that the aforementioned lady, maiden name McKelvie, was not the inspiration / stimulus for Iain's song!
(apart from anything else - the girl was not called Annie or McKelvie)
Many of Iain's songs begin with a small incident or remark - - but the song usually then takes its own direction, it is not the detailed story of that particular occurrence!
Some of the historical songs are accurate (e.g. Quintin Hill, A Bankie Lad, Ill Wind Frae Cumberland) or as accurate as Iain can make them from his researches; but like any writer he may then introduce a character who personalises the experiences -- without that character ever having been a real person!