@Kotalier I like the observations made, although I feel like a lot of other factors of the story are being dismissed in Miku's favour. As donm527 mentions, the story/themes that have revolved around Miku have been about her love towards him, and in the latest, it's been about her ending her relation as tutor/student with him and have one that is eye-to-eye. Unless the author is really hiding her deeper depths, the focus she has had would give her too many things that she is dealing with. If the statements she has made about love over the chapters are to be true, (yet again as donm has already stated), her being "Rena" would be an advantageous tool.
Certainly, Miku has been a shy one, but I feel that you read too much into her actions. When she was averting her gaze and being a pace away, it could have been just her personality as a shy individual. When she was close to Fuutaro in the waters, that could have been the author choosing a girl to have a little good tease. To my understanding, Rena wanted to become better, but Miku at the start wasn't exactly striving for that; if she had given up on it... well, among the five sisters, she seems the least driven to have the sisters be together (not that she doesn't want to be together, but she is showing off to be the least proactive). Some may say that it is a front, but then the same can be said for other characters.
I'd actually hypothesize that it is still Yotsuba. Even though when Rena showed up, Yotsuba was supposed to be on the run, but we haven't been given a definite time frame of how long she was away from the running group. Of course, why would she carry a dress and wig around, we are not certain, but if you state that some things are not determined finitely, I'll also mention the others that don't seem so clear too.
She is also "the least developed"... not because she lacks development, but the author doesn't place too much attention to what goes on internally. The others have gone through their own journey, but Yotsuba has been a wild card. This is also the possible reason why people don't see her as Rena because not much has been offered about her, thus there is less a strong impression upon her unless an individual likes her off the bat whereas most others stick her in the "side heroine" position... to my dismay. But that's life. But just like yours, my theory is but a theory and I could be wrong.
@nwatn any of the girls could have had their ears pierced... heck, they may have pierced it for the wedding. imo, the "earing" idea would only work when identifying Ichika in the present, not in the future.