@du5k Some of your points are definitely worthwhile considerations, but I feel others are flawed.
1. She the primary deuteragonist,
I agree that this is in ways a reason, because stories after stories have had the main female lead be the "mystery girl." But the same can be said of any other heroine, as other stories have non-central heroines be the "person of the past." Heck, even one major romcom of the last five years, Nisekoi, did this; it doesn't rule this possibility out, but it's not a strong one reason especially if Negi wishes to break convention.
2. She says the same thing as Rena in flashback,
I can get behind this. The chances of a present character saying almost the same thing as the one said by someone before isn't high unless the statement is a very common one. I'd argue that the moment Itsuki saying it didn't have much story drive to it, but still, the possibility.
3. She's super interested in the reason why he studies so hard,
I don't know if this is a reason to pin Itsuki as "Rena." Certainly, Rena has shown interest in Fuutarou's coming future, but I feel that it's possible for any one of the quintiplets to just be interested in why he studies so hard over the course of the chapters, not because of some nature of the past. If anything, it could be a sign of Itsuki's actual friendship to Fuutarou.
I'd even say that if "interested in him and why he studies hard" is a prerequisite for being Rena, this could be applied to one other girl: Yotsuba. Miku, Ichika, Nino and Itsuki from the start showed no interest in being taught, whereas Yotsuba did. While not an obvious sign of interest, but maybe one's interest in someone's hard work doesn't have to be so outspoken; Yotsuba, if Rena, could be feeling relieved that Fuutarou is studying so hard that she is impressed by it (as seen at the start) and even wants him to teach him. It would help if she did display actual fondness of his hard studying, but I don't think it's necessary when establishing a character as "impressed by/interested in someone who studies hard."
4. She's the only one who likely know where his location,
This one does get me. Who else would know his location? Some like Miku are a possibility, but only Itsuki is the most certain one. I wouldn't rule out others unless they are like Ichika who is actually busy with work. (I also disagree whenever anyone states that Yotsuba is like Ichika because, while she is commited to helping the club, we are not given any clear time-stamp of how long she was away; her commit, unlike Ichika, is not a job, and if Yotsuba as Rena had priorities to meet Fuutarou, she might have considered ducking out for a moment from club activities, along with creating a supposed alibi for us readers to be fooled by).
5. Rena refuses to take off her sunhat, making me think she has something to hide, like a giant ahoge,
If I remember correctly, one of the post-pages in the volume for that chapter shows a set of hands putting the dress, sunhat AND a wig into a drawer. If this is the case, I don't think hiding an ahoge is a problem as the wig would take care of it. I also think that Itsuki has more than that ahoge to worry about when it comes to her hair, because look at the size of those tufts of bangs that sweep to the side.
6. She has a motivation to make him forget about Rena,
I think if any one of the girls was Rena, the author could write for them a reason for that girl for Fuutarou to forget about Rena. As we know, Rena's wish is for him to not look to who she was, but the current quintuplets. Some are a further stretch, but Itsuki is not alone in having a possible motivation.
7. Negi needs a "hook" for Itsuki to remain relevant amongst the sisters for now, and she doesn't yet have her big arc,
If the author needs to keep a character in with a hook, especially one who is supposed to be the centrally advertised heroine, that's author neglect. I do want Itsuki to have her own arc, but having her trump card be "I am Rena" is... well, to say the least, a poor choice. The best outcome in writing the potential Rena heroine would be writing her with more context that makes her a standalone character, and for now, Itsuki is behind in that race.
8. When Fuu asked the girls if anyone seen him when he was younger, the other 4 girls doesn't really have a reaction and Itsuki's reaction is not shown in that shot,
I don't know about this reasoning. As part of deductive reasoning, this is a scene to consider, but maybe it's a false accusation to say "lack of inclusion = suspect." Certainly, that is how detective novels work at times, but who knows if one of the girls is acting it out. Others have even argued with this same scene that Miku/Ichika is Rena because of how they reacted (averting gaze in Miku's case, good actress for Ichika's).
9. I don't think Ichika or Miku will receive so much character development if they had such a big plot in the story, and no active motivation to do,
I do agree that since these two girls (I'd even lump Nino in since she's been having just as much attention lately) have new/concurrent motivations that would collide with Rena's. Either that, or they are purposely having different goals in hopes of fulfilling both of them (for example, any of the three may want to win his heart purely by present-them and without the help of being "Rena"). Their focus has been about their romance/potential romance with Fuutarou, and it might dilute their quality if "I am Rena" was stacked on top.
10. Still don't think it can be Nino since she's pissed with him atm, and won't have reacted to Gintarou like that,
Yep, Nino was at the time in a hostile relationship, marking her off as the least likely to be Rena. The chapter after too also would have been way to awkward. Some cling on, and I've even heard one say "It could be a dark horse, where she was acting all along"... but, Negi has somewhat already pulled off enough of a creator's pet. Unless Negi is fine with being thought of having favourtism to her, I'd expect him not to pull the "I was Rena" with Nino at all.
11. nor Yotsuba who's with the track club atm
As mentioned above, I feel "track club" has been a loose excuse to remove her from the possibilities. I could imagine any of the other girls, if they were in a club, leaving momentarily to act as Rena to approach Fuutarou, later appearing as themselves as their club friend comes to help create an appealing alibi. Yes, I am defending Yotsuba extensively because I believe her to be Rena, but even when putting that sentiment aside, I feel that too many people chalk her up as immature and unable to fool people... which does play by her character, but I also feel that there is more to her that may reveal that she isn't a true "dumb jock."
But yeah that bush thing is really throwing me off.
If I was in support of Itsuki, I may go for the "it was not actually anyone." A weak gimmick, sure, but maybe focusing too much on the bush thing detracts one's attention from other details. Red herrings, right?