but then the conflict aspect in 2nd half was kind of "eh"
Yep. It got all messed up and lost the tension and sense of strength in the different factions. At the end it became a kind of Marvel Avengers spectacle and Nagi
I think the show was great in the first half at setting up gloom and doom by keeping some politics in play
You really felt the factions were stronger at the start, but a stronger writer would have managed the 2nd half better to keep the stakes high. The tone shift with Mitoko (girl, not cat) and the Black Valley and other elements, affected the sense of intrigue. There were too many elements that just came out of the blue and a lot of things weren't explained sufficiently to give them weight when they came into play. Also, almost no known character died again.
I loathe the Sergay, Nina (and Arika) dynamic. LOATHE.
Yeah... The story straddled the line, definitely, hinting and taking things to the line but always stopping short. Sergay stopped at the point of taking advantage of both of them even when they willingly offered, but the writer always let us assume that he had done it before subverting later.
Looking at the broader context, Nina and Arika were 15, right? So, just about to hit the age of consent in Japan. I could see that was why the "all-consuming first love" theme was so emphasized as a pivotal plot point for both of them and in the story (also reflected in Akane's desire to pursue her love instead of becoming a Meister).
Nina and Arika's love stories might be seen as a crush for a teacher or something (he served as a mentor to both of them) amped up to a 100% under dire circumstances. You can see that the 17-year-old Midori in comparison was always painted as a young adult.
Nina and Arika were beginning to realize their latent powers and destiny and coming to terms with their heritage (who their mothers/parents were). This was also juxtaposed with their friendships, self-actualization/selfishness, and so on.
Sergay wasn't really needed in the story except as a first-love interest and amplifying factor for their desires/powers. They both reached their full potential in part because of their intense focus on him.