I love this chapter. First off, I'd say it's pretty clear that this chapter is about depression, about the darkness that fills a soul when darkness fills one's life. The gremlins are the voices of accusing people or thoughts.
I think the core of what happens here is possession of the Marble Lady. At first, what the protagonist says is, "I no longer called her MY Marble Lady" because he was losing hope. The voices of the gremlins pick up on this thread when they mock him, "you shan't have her." She will not belong to him, she will belong to another, better man.
After that, just barely is the protagonist willing to give up his claim on the Marble Lady: "It is well that thou should'st be, of the nobler, bride."
To give up your own claim to the Marble Lady, after such long and fruitless seeking, is certainly a "spark of nobleness" as the author proclaims.
The point of the second song is similar; by trying to greedily take as much as possible from those things that give you life (a violet, lady's gracious eyes, and "the maid" respectively), you end up killing the source of life and deceive yourself as well, while doing wrong.
The meeting with the elderly woman draws on a different theme, illuminating the depression. From this episode, the reader can discover that this whole country of bare rocks, gremlins and darkness is really just the product of the protagonist's depression over the lost Marble Lady. What the old woman says is that the companionship of any "pretty girl" would be enough to break him out of his depression. When she is transfigured into a beautiful woman, she similarly transfigures the landscape to a beautiful countryside, because the countryside itself is a product of the protagonist's mood. That's why he always chooses the lower paths whenever he had a choice, because the darkness within him was seeking to find darkness without.
The protagonist, in his turn, turns down the woman's claim and continues his search for the Marble Lady (not that he expected to find her).