Keitaro and Nagisa becoming so close to each other throughout the story is a reference to the story that the original series was based on. Nagisa’s name refers to a shore where Taro came across a turtle that allowed him to find happiness. It also make sense as Nagisa's name means "someone who help others", which Nagisa has done in the story.
Why is Amalla so insistent on blaming Keitaro instead of her sister for what she did? Because as Keitaro pointed out, Amalla thinks exactly like Su does, with complete nonsense.
Fridge Horror
This story demonstrates Ascended Fridge Horror when the antics of the Hinata tenants are treated seriously, such as Su's idea of "fun" involving destructive machines and using them on people regardless of whether they can handle it or not.
