This series has become very quickly in of my favorites, the art is gorgeous and the writing is surprisingly good despite taking a somewhat overused premise.
Now as for the chapter itself, unlike many people here, I think the possibility of a threesome ending was dashed a bit. Whereas Lapis' growing affection for Natori is being framed as a chance to save her from her self-destructive plans and maybe even help her to find redemption, Diana is being framed as somewhat toxic and pushing the poor girl into destructive and harmful behavior, I wouldn't be surprised if Diana's character ends up being about learning to let go of her affections for Natori so she can become the heroine she's meant to be.
Lapis is by far the most fascinating character in the story, there is a great balance in showing her as a confused and somewhat misunderstood teenager but at the same time, there are subtle hints to show that she is a villain. I read her interactions with Diana as her stating her claim over Natori under that mask of politeness for no other reason than to be petty.
I'm surprised about all the people assuming that Rubeus is acting on his own since I think that he is acting on Gran's behalf to try and stop Lapis' plans before they can start, I think that the implication that Diana could claim Natori for herself if she helped Rubeus was an obvious hint of Gran's involvement.