I really enjoyed the lyrical imagery in this story. It drew me into the story, and I was all but shivering with the MC when he was fording the icy river and braving the snows. I admit that I usually don't like such an abrupt ending with all its untied threads, but I figured it must be a French thing, and it kind of worked for me for some reason.
For me, I envision the Coalwoman as a nature spirit like Circe from Homer, or a Fae of the Norse mythology. Coal represents ashes and soot, the burning destruction of things, and the thing that we all become when we die. I think, therefore, the Coalwoman represents Death and Dying, her coals are the Souls of living, and the boats represent Charon (the ferryman of Hades) carrying the Souls to whatever fates await them. Once he entered the Coalwoman's world, the Knight enters the Coalwoman's kingdom, which I think is the spirit world, or the world of death. Maybe he died already when he first attempted to come onto her island, and maybe he existed only as a spirit (which explains his initial youth and vigor), while the Coalwoman slowly fed on his life-force as evidenced by his continual aging.