Merry:

There is a glossary of Scots words here in the Forum:  Click Here  which you may find helpful with some of the less-familiar words.  In versions 2 and 6 -both Scottish- the point is that the older woman thinks that the beggar is an old man, but he's really just disguised as one, as the final verses go some way to explaining.  "Maid" can mean a child, a young woman, a virgin of any age, or a female servant; in this context, we're obviously looking at the young woman variety.   As to the refrain, I don't think that there's likely to be any hidden or lost meaning; quite often these things contain just enough "real" words to sound like a proper sentence, but really are just nonsense.  An example from the French tradition would be La Belle Fille de Parthenay, with the line J'ai me lonla lonlan dérirette, j'ai me lonla lonlan dérira.  As for mythic interpretations of traditional songs; these were very popular in the late 19th/ early 20th centuries, and were indeed highly entertaining.  All that talk of Sun Heroes and Sacrificial Kings...most of it was pure romantic speculation based on very little genuine evidence, and such theories are rather out of favour nowadays.  There's no harm in a bit of speculation, though; sometimes real insights are to be found that way.

Malcolm