So here's my take on things:
1. Yotsuba was Rena in the past, the girl who Fuutaro first met. She was the one in the photo (which now we know was taken in Kiyomizu-dera), and also the one who bought all the charms.
2. Ichika was the girl playing cards with Fuutaro, I think this one is pretty much confirmed. She did that after Yotsuba told all her sisters about Fuutaro that she had met.
3. Fuutaro remembers that the charms got washed down in the river (Chapter 82). But we do know that the sisters still all have the charms, and in chapter 83, we see that Ichika doesn't have any clue about the event.
4. Disguised Rena was Itsuki all along, from the first appearance till the last. She was the one who heard Fuutaro's story first. It's likely that at some point after that, she remembers who gave the sisters all the charms. So she knows that it was Yotsuba.
5. At the first appearance of Disguised Rena, Itsuki was trying to make Fuutaro let go of this past Rena. But afterwards, she doing just the opposite, she's trying to make Fuutaro remember. Why this change of heart? Does Itsuki know something about Yotsuba? Maybe she and Yotsuba had a conversation, and now Itsuki was trying to support Yotsuba?
6. Related to point 5, who was the bell kisser? Nino is unlikely, she wouldn't want to kiss him under disguise. Ichika is also unlikely, and someone already pointed out the sprained ankle, so she couldn't have been able to run to Fuutaro at that moment. Miku had just got somewhat favorable event, so i don't think she was that desperate for the kiss. So that left only Yotsuba and Itsuki. But looking at how Itsuki behaves afterwards, I don't think it fits. I think Yotsuba is the most likely to be the bell kisser. Itsuki though, might witness them.
And unlike Fuutaro, she wouldn't have any difficulties to recognize which sister is under the bell. Maybe that's what brought the change? Now she wants to support Yotsuba.
7. Now back at the first Disguised Rena. She was telling Fuutaro to open the charm when he was ready. I had a theory that after the charms got washed down, somehow Yotsuba made all the replacement by herself, and the written text inside was all from her. Maybe Itsuki opened it, and realized how much important this young Fuutarou to Yotsuba? That's why she told Fuutaro to open it after he "acknowledged" himself.
Look at the Kiyomizudera scenes again. Based on Yotsuba's reaction, I think she started to realize what Itsuki is doing. Next chapters might reveal some conversation between them after that event.