I have to say I'm very much leaning towards the idea that Nayuta is the same as Mahiro as well. While I resisted the idea at first because it seemed like such an easy conclusion it had to be a red herring, there's just a ton of small signs hinting it.
-- Compare and contrast Mahiro and Nayuta's body language. The latter is a lot more stiff while Mahiro's is just incredibly girlish now. You can really see it in this chapter in particular.
-- Nayuta was squirming quite a lot when stared at by the other girls during the physical exam. That could just be from being shy but Nayuta seemed to be kind of quietly fidgeting throughout the chapter based on their facial expression.
-- Nayuta's got no friends and is incredibly awkward socially. This is a very similar situation to Mahiro's start. And I mean it's beyond the pale if Nayuta thought it was a great idea to give Mahiro the panties they were wearing.
-- While not in fact conclusive, Nayuta did call Mihari's senpai "onee-san". That doesn't have to mean she's her literal sister, but in the context of everything else...
I'm starting to suspect the two of them are experimental prototypes of a larger "turn awkward nerds into middleschool girls" project. So if any of you wake up one morning and you feel... different... now you'll know why.