Everyone's guessing which Itsuki is which girl but I'm here guessing that the Itsuki Fuutarou talked to over the fence is a different Itsuki from the one who barged into the mixed bathing section simply because of the sheer speed that makes it pretty unlikely.
Also, when did they all get a hair job to make their hairstyles alike Itsuki's? I can never get my hair any other shape than my current hairstyle.
Oh wait I forgot that wigs exist which would explain how Miku and Ichika's hair can be as long as Itsuki's... but then where did they get the wigs in a public bathing house?
I think the Itsuki who called Fuutarou the other day is Ichika (because she heard Nino's confession, so maybe she's REALLY akward), and:
- Nino is full mode savage right now, there's no reason to pull that off.
- Miku is best girl and loves him very much, no reason.
- Yotsuba has always supported Fuutarou.
- Itsuki said she has no idea about, so it SHOULDN'T be her.
Pretty sure it wouldn't be as easy as reading their facial expression. Miku can change into Yotsuba at the drop of a hat while being chase in chapter 4 iirc