Kuina is not dead, she was taken back by her Celestial Dragon parent, same as Shuri. Zoro's master decided to give her up, rather than fighting like King Reuven, in order to protect the village and the other students. The stairs were a stupid cover story that you would tell to a child, who would be unable to understand geopolitical politics. The body in chapter 5 is a fake, you can see that they do not want Zoro to go anywhere near it, and Zoro's master does not look too upset standing next to his dead kid. Rather than Down D. Stairs, she went Out D. Door, and Up D. Mountain. Zoro's final fight will be against the World's Strongest Swordswoman, where he needs to beat her with her own sword, and her haki inside of the Wado Ichimonji will resonate and turn the blade black.