As for sunlight, it can’t be just that sunlight kills vampires. That vampire in the school endured 10 years of hell starving himself for the chance to die. If he could have just committed suicide in the sunlight any time, he would have done that years ago. His death had to have been caused by the ring thing the hunter gave him. Further, when Kou told Hatsuka about the guy’s death and turning to ash, Hatsuka said something like “Turned to ash? Could it be...?” implying that the sunlight death wasn’t usual and was the result of something specific.
Maybe that’s what Nazuna’s been hiding? That she actually can go out during the day, she’s not confined to the night like Kou thinks? She might have held that back because it’d shatter his ideal of the magical night time when the reality is it’s not special at all.