Robert Graves was a British poet, an expert on Classical and Welsh literature, whose hobby was detecting in ancient cultural artifacts the "fossilised" relics of even more ancient cultures. If evidence was lacking, he would occasionally use his poetic intuition to reconstruct it--bad scholarship, perhaps, but good poetic practice, at least according to Graves. (By the way, if you want to get up the nose of a Spiritual Feminist you can try to tell her that Robert Graves, a man, thought up all this goddess fal-de-rol back in the thirties.) I'm not sure if Graves invented the concept of iconotropy, but he certainly made great play with it. The above riff on Three Ravens was perpetrated solely by me, following Graves' example. It's 99.9% BS, of course, but it points in the direction of one possible back-story for the ballad.
Like Liath, I think there must be folk-tales on this theme of the deer-bride. I half remember reading a story about a hunter who encounters a person who is his prey in human form. And I think she is a deer, or were-deer. This is a tale of magic and enchantment, not a vehicle for run-of-the-mill figures of speech. But what is the song as we have it now really about? I agree with Tim, it's about love, faith, and bereavement. Speculating on the back-story is fun, but the song speaks for itself.