I wonder... is there a limit to the amount of time she can rewind back to.
A few chapters ago she gets the watch in the spring of her second year of highschool. Now this chapter
ends with her returning to the spring of her second year of highschool.
So that is my guess is that point in spring, in her second year of highschool, is the earliest point in time that she can go rewind to. She can only go back to when she is in hospital. She can't prevent the family's initial downfall, stop the middle school bullying or fix the first year of highschool. It is a good limit, I think, because otherwise just find Clover as a baby.
Also Fuu helps Yotsuba discover Yae is one of Uichi's tormentors...
It is interesting, because we did see that, but I believe
it is implied at the same time that the Fuu-chan who outed Yae is from a timeline that was replaced before we even started the story. If that holds true, the current Fuu-chan we get to see isn't the same one who outed Yae to Yotsuha, so wouldn't have been able to plan around it. The Fuu-chan we have seen has only been asked to track down Clover, a task she might be dragging her feet on. And after Yotsuha got her involved in this timeline, Clover started asking about "Uichi's friend" unprovoked.
I think Rei-kun being Clover is a misdirect. I see no explicit information to assume it is Yotsuha's brother other than him being a character that exists. I might drop the manga if it introduces the level of time-travel shenanigans two competing time rewinders would involve.
I think most of the information we have available at this chapter heavily implicates Fuu-chan being Clover. She knows Uichi early enough to form a weird grudge before he starts middle school. She shows a high level of affection towards to Yotsuha, similar to how Yae did and maybe enough to adopt her clover motif. Yotsuha leaked snippets of what is actually going on to her. She was shown to have a superior intellectual ability even as a child, which was probably enough to manipulate a slightly older sister and destroy a family. She has the computer ability to do everything Clover has done. In a lot of her scenes, her reactions are purposefully obscured from us by the mangaka. I think we never get to see her internal dialogue.
But because it feels like the mangaka wants us to think it's her, it is actually probably not her.
Ultimately there is purposeful misdirection and the very real possibility the mangaka doesn't even know the answer as they're pushing out chapters. Their income depends on keeping this going as long as possible, so it is more likely we are going to get a handful of new characters that could all potentially be Clover than it is that we get answers anytime soon. It is very very fun to speculate, but ultimately pointless.