Your original question had to be edited as it actually contained three separate questions. Please note that enotes regulations state that you are only able to ask one question, and that no multiple questions are allowed.
Focusing on the imagery that is used to explore Sylvia's relationship with nature, you might want to think about the part of the story when Sylvia climbs the old pine tree in order to try and locate the white heron's nest. As she climbs up and looks out at the view, consider how the sight is described:
Yes, there was the sea with the dawning sun making a golden dazzle over it, and toward that glorioius east flew two hawks with slow-moving pinions. How low they looked in the air from that height when before one had one seen them far up, and dark against the blue sky. Their gay feathers were as soft as moths; they seemed only a little way from the tree, and Sylvia felt as if she too could go flying away among the clouds.
Think of the way that Sylvia identifies herself with the sight that she looks at. She feels a strange kind of kinship with nature, and desires to be like the hawks and to join the sight that she is a spectator of. Sylvia is clearly described as a character who is open to the living, breathing beauty and magnificence of nature, as the imagery employed in this paragraph shows. This of course symbolises the intimate relationship she has with nature and her eventual decision to disappoint the young hunter rather than betray nature.