After finding many versions of this song, and seeing that it was first obtained in 1665, I have come to believe that a lot of different people have added to this song.

I came across a verse that reads.....
"Oh mother dear, you caused all this
You would not let me have him
I might have saved this young man's life
And kept him from hard dying."

Back then it was a lady's honor and her family's reputation that something like that would have hurt. A Mother would have persuaded her daughter that he was not good for them.

As for the unknown verses, it was comman to add to a song you learned. When it changed languages and origins, verses and words changed. If a tune was catching, but someone didn't know all of the words, then they may have added or changed words.

Best way to find out, is to research scottish archives.