I've been thinking about your question about how Ellen has power over Newland. In terms of the relationship between them, Ellen is certainly the more powerful individual. Though she is a married (though a permanently separated individual) and Newland was technically still "free" when they met, he, as a gentleman, had a duty to May that she would not have if the situations were reversed. One of the few things which mitigated the double standard of this time was that women were perfectly free to break engagements without anyone reproaching them, while men were considered "cads" if they broke an engagement with a woman (barring that she wasn't disgraced or lost her reputation in some way.) In engagements men were held bound, but women were allowed to change their minds much more easily. Therefore, even though Ellen was married and Newland was not -- and, in most situations, Newland would be the in the more powerful position -- it was the reverse. Ellen had a husband who was far away, and everyone agreed he had driven his wife away by his shocking behavior (infidelity, etc, though it's not expressly stated except once, in private -- to another man -- by Newland). Ellen, though tinged with scandal by using a man to help her escape, nevertheless was the wronged party. She came back to her natal place, and lived with her eminently respectable Granny Mingot. Though not free to marry, she was still a single woman with no father, mother, brother, or husband to tell her what to do. Granny Mingot certainly would want Ellen to behave properly, but having come from an age (the earlier part of the nineteenth century, before the Victorian era) in which some social standards were looser, she had fewer strictures on her than if, for example, she had been under her aunt Mrs. Welland's direction. Ellen was to all intents and purposes free. Newland was bound by his own traditionalism, and the expectations of his family and his class to stand by May no matter what. In the end, even though Newland is willing to leave May for Ellen, she does not allow it. The chivalry of the time demanded that Newland do as Ellen asked.
It's a somewhat tough proposition proving that Ellen had power over Newland, because she so seldom chooses to exercise it. In the end the one who has the power is Newland, though only briefly. He does not come up to see her in her apartment in Paris -- he refuses to let the ghosts of their past relationship taint the memories of his marriage to May. But the relationship between them was never one much concerned with power (unlike many of the relationships in this novel.) That was part of Ellen's charm for Newland -- she had no demands or expectations of him. For him that meant he was willing to give her everything -- and he was fairly certain (and correct) that she would never take it. It is a relationship about feeling and sentiment, not about power, in my opinion, and that's part of what is so poignant about the novel's ending.
Hi danomitex -- see my answer in the Q and A. In addition to the things I brought up in answer to your question, consider Ellen as a character. Specifically, consider her in comparison to the other women in the novel. What does Ellen do that no one else does? She crosses rooms to talk to men, for example, like she does in that first dinner at the Van der Luydens. She arrived late, fastening her bracelet on her wrist as she walked in. She has an air of power about her, (re-read that scene, which, I believe, even says that she has something like "an air of conscious power" about her) and she knows her own capabilities. Ellen seldom -- if ever -- does things out of fear. At the opera that first night, she refuses to go to the ball because her dress "isn't smart enough" -- but Newland discovers later that it had nothing to do with that. Ellen wasn't afraid of anyone criticizing her dress, but she didn't want to overshadow May's big night. Ellen is a fearless character in many ways, and in the New York society in which she lived that made her much more powerful than the people around her. The society they constructed was based, at least partially, on a fear of appearing different, immoral, or common -- and Ellen is not afraid of appearing as any of these things. This lack of fear on her part makes her more able to act on her desires, but also on her own sense of morality (and not society's). In this she is most like Granny Mingot, the only character in the novel she is the least bit like.
Thanks so much for your replies sfwriter