Rhysand is probably the best example of duality. He has two conflicting sides, metaphorically represented by the Court of Dreams and the Court of Nightmares. He is shown as brutal and heartless is the first book. His place of residence, the Night Court, is described and depicted as a place of hell and torment (in the first book anyway). Rhysand is the embodiment of this place and its values as he is their High Lord. Whilst, in book two, we get to see Rhysand's second side - metaphorically represented by the Court of Dream. He's a family man, arguably depicted more as a hegemonic male instead of a cruel dictator. This depiction is kinder than what we get to see in Book 1. However, his crueler side still exists, metaphorically represented by the Court of Nightmares. He is still a vicious animal, and for a long while attempts to hide this side from Feyre. I remember in Book 2 or 3, Feyre tells him something along the lines "I'm not afraid of you" suggesting that she loves both his Court of Dreams side and the cruel violent Night Court lord he also is.
Many of his actions within Book 1 are also examples of duality. If I remember correctly, Rhysand kisses Feyre. To Feyre (and the readers) this comes across as perverted - he kisses her without consent and when she is clearly in love with Tamlin. However as revealed later in the series, he kisses her to stop Amarantha from acting suspicious (or something along that line).
Your points with gender are good. Also remember, A Court of Thorns and Roses is a fairy tale retelling. Traditional fairy tales have an interesting representation of gender. Typically, women in fairy tales are Proppian archetypal princesses whilst the men are warriors and heroes. ACOTAR does subvert this; Feyre is the hero. Maas's world in the series is also depicted as a patriarchy. High Lords (who are all men) are in power. Feyre inevitably subverts this by becoming a High Lady though.
Hope that helps. :)