I guess I get this now. Miya started to get weird because she thought Nachi's change into a gyaru was some sort of 'play'. Miya started showing Nachi that she would basically be fine with any sort of play that Nachi might be interested indulging in. A very miscommunicated way of telling Nachi, "No matter how you change, I will always accept you."

Nachi, who was just trying to create some distance. Got confused by all Miya's craziness, but accepted it very quickly after realizing the reason for it all was because Miya wanted to stay close to her. Nachi also came to understand her childhood friend is insane and has to keep an eye on her.

I accept this interpretation. Wholesome!