Throughout the story Feyre is forced to shed her feminine side and forced to adapt a more masculine role. This happens multiple times throughout the series and is important to note that she often shifts back and forth between the masculine and feminine. To give an example of what it is I am referring to; once Feyre's father runs out of money and they move to the hovel where the story begins, Feyre is forced to give up her life of studying and being a normal girl and must assume the role of the protector for her family. In the world today this is seen as a masculine identity. She goes out and hunts and provides for her family. This is the first example of Duality that I noticed. I am a college student writing on this topic and would appreciate if anyone has anything to say or would like to discuss this topic in more detail.
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. :)