@donm527 Much appreciate the pointers. Can't get to every detail... or I just need to do my own research better. I am better convinced that Ichika was meant to fulfill the insecure older sister role, and that her moments of weakness have been shown as that gap between "reliability" and "vulnerability"... although I still find some of moments with her jarring (mentioned in my response to Nono). May just be because I haven't found characters like Ichika (flirty/carefree, but actually lacking in certain certainty) to be my appeal, maybe...
Even then, I do admit to not being convinced of your absolute statement of "Ichika=Rena." She was scouted, and she road the opportunity, yes. However, as mentioned, Itsuki also wanted to be a leading figure. It feels then that the two at the least took different approaches to "changing" in order to support the family: Ichika took to doing it financially as the actual eldest sister, and the one doing what she can to realize the others' needs; Itsuki on the other hand believed herself to be the most level headed, thus the more... emotional support? As mentioned again, young-Rena wanted to "set an example" and also stated how Fuutaro was necessary to it all... but with the two above, that does not exactly apply based on how Ichika and Itsuki have interacted with Fuutaro. Of course, I could be jumping the gun and may be assuming that young-Rena needed Fuutaro for the years to come. Something just does not click.
Even if Ichika was trying to be a leader, young-Rena does not fit into that description. How she acts wouldn't match with the bit-bosy old Ichika, neither what could be assumed to be her new responsibility of "responsible leader." It doesn't mean the other four, if they wanted, wouldn't want to do something for the other sisters. We have enough to say for the other three (Miku shows the least sign that she had wanted to change five years ago in reaction to their mother's passing) would have done what young-Rena had done.
@KoreJi Thank you for bringing that insight over here. Even if it was not your insight, it helps get ideas across that may be missed when insulating discussion to just here. Certainly wondered how Yotsuba's statement was being said in the original Japanese, since we all know that there can be much lost in translations, but I hadn't had the opportunity to search online for the original raws.