Typically, "spey" or "spae" is "the opening or slit in a gown or petticoat, etc." Willie didn't even manage to get into her underwear if we are to take the definition usual in Scots dictionaries; though perhaps MacColl or his source had other ideas.

"Hire" here is probably "to accept, welcome"; particularly in the context of a dealing or trading transaction (see for example Alexander Warrack, Scots Dialect Dictionary), which is essentially what the Scottish custom of marriage-by-abduction was.

So far as other interpretations go, remember that the text under discussion here is Ewan MacColl's. He published it in The Singing Island, 1960, 32; saying that it came "from the singing of William Miller of Stirling and Samuel Wylie of Falkirk, Stirlingshire." William was Ewan's father, of course; and the Miller family do seem to have been unusually fortunate in their possession of rare ballads, and melodies for them, unknown elsewhere.

Bronson, Traditional Tunes of the Child Ballads, III, 361-2, quotes text and tune, adding: "Mr Ewan MacColl is the sole authority for a tune of this very spirited, if brutal, ballad of bride-stealing. He learned his tune from his father; the text has come mostly from Maidment" [North Country Garland, 1824, 40)] "by way of Child, with some help from Samuel Wylie of Falkirk" [presumably the three extra verses]. "I am unable to point to any analogues for the melody, which does splendid service in MacColl's vigorous rendition."

Of Maidment's text, Child notes "He does not tell us where the ballad came from, and no other editor seems to know of it."

Maidment felt that the ballad was "evidently founded on fact", but failed to trace it to any historical event. It's unsurprising if there are inconsistencies in names and so on, and there may not be much mileage in worrying about them. As like as not it is pure fiction, though, as Maidment also observed, the circumstances which gave rise to it were common enough.