@takashiobatasan That's cool, I can understand not wanting to look into the author like news or twitter in case of spoilers if you're one to avoid them.
There is a chance the author could introduce more memories to show other sisters clearly saw him 5 years ago... maybe after Itsuki told the sisters she spent the day with a boy it could have led to all the girls taking turns seeing who this boy was. But again I doubt it... maybe.
The sister walking with Fuutaro in the evening... Chapter 83... it was Fuutaro telling Ichika his memory of his trip in elementary and spending day with Rena. It was also how he tripped up Ichika when he asked if she remembered buying the charms together.
Re-reading it... it could be him telling Ichika of his day with Itsuki and being led by the nose by her until it got dark... and then he separates from Itsuki, she tells the sisters of her day and that Fuutaro is still in the lounge... and then it's Ichika showing up and spending the evening playing cards. So maybe there wasn't a separate evening event but Itsuki spending all day wandering until it got late into the evening. Yeah, I'm gonna add Yotsuba to the list of 3 that didn't see Fuutaro back then... for now.
Chapter 85 confirms Ichika saw him when she'd going through the flashback of seeing him in the banquet room, playing sevens, a flashback to the test of courage seeing Fuutaro with white hair (I think first time realizing he was that boy), and then confessing to herself even if it was that short time that evening that (she) came to love (him). That personal confession to me is super deep compared to all the other sisters and connects a lot of the feels the two of them had through all the arcs. So that's why I really feel it's Ichika... Can't see the author sinking such a deep love for Fuutaro
There are still holes that need to be filled... like the bell as you mentioned. That will be another piece that may clear things up hopefully soon.