@Kotalier
I am in ways honoured to be asked for an opinion on one of these comment threads, since the expected flow of such thread is either arguments, discussions or simple tag-alongs. Asking feedback to me a good sign of discussions turning towards... well better discussions as people who converse are being more open to what others might have to say and their validations.
Anywho, away with my appreciations, I do not mind giving my thoughts on your findings. None of us will have a true right or wrong, as it is more about more convincing evidence over those that are superfluous.
Not to discount an observation off the bat, but it feels like the "experience in playing a non-quint" is part of my hunch, or better to say spiteful peeve, that Ichika's acting career gives her a lead... which, to remain honest, is an argument that can be made. However, among reasoning, I've usually resolved this quality to be more a supporting claim than something concrete. as we are not entirely familiar if any of the sisters have any form of acting skills beyond masking up as one or the other. Certainly, the other four don't have much of a reason to know good disguises... but if a non-Ichika quint expected to meet Fuutaro eventually and her looks didn't make clear connections to "Rena of the past," I can be certain to a degree that quint would have readied something. Let's also not exaggerate the skill needed to think of a potential "what if" idealization; I'd reckon that the Rena-present, be it Ichika or not, would have put two-and-two together to change things up a bit: make herself look more "modern" than keeping to the white one-piece alone. Or who knows, maybe she could have kept to the old look, although that'd look rather... how to say? Incomplete? When you think about it, while it does make it feel like you are meeting someone from the past, a person wearing the exact same attire gives an impression that they are dead... which as some of us takes, Rena of the present does want to declare "Rena of the past" to be dead, or at least someone Fuutaro should not get too caught up in.
TLDR: This reasoning has been what I call a supportive reason, not a main reason to support Ichika being Rena. I'd take a guess that any of the sisters could take up acting, and probably would have done moreso if they are Rena. Yes, a slip is possible, but from what Negi-sensei is trying to display, Rena is able to act well (and is trying not to make Ichika's acting an obvious clue) with obvious moments where the acting does slip such as when she blushes (face downwards) to Fuutaro's compliments of the quints.
Two. Yes, this point I can agree with. Of course, Rena could have been lucky in how she was able to run (I think him slipping off the boat was luck), but it seems more fair to believe that Rena is intuitive in making on the fly decisions. Some may argue that she isn't as crafty, since some of her quick-decisions are not as pre-planned (I wouldn't exactly call her as quick-witted when she was saying "how do I stop you...?" right before riding the boats). Therefore, we may be giving more credit to Rena than needed for the "running from Fuutaro" part, as that could have just been the author giving her a dramatic exit.
Three, at first, I found this finding to be intriguing. Handiness in some mystery series can be a lead to something, so I checked back myself on the chapter... but, to my finding, I'd say that this argument isn't as strong as it was initially. The reason why I say this is because your point about her left-handed usage being an indicator of some degree of ambidexterity... is lacking.
When you observe an individual, a person's off-hand can follow a handful of trends: it can be weaker but more dexterous; it can be stronger but not so dexterous; it can be like a second dominant hand. But in truth, you don't need to be dexterous/ambidextrous to use your offhand in life. If you don't mind doing a bit of an experiment, keep a vague note in mind how many times you use your off-hand; also ask the same of some people in your life, be it a friend or family. You can say that you will come to the result that people who aren't ambidextrous will still use their off-hand in some tasks, consciously or unconsciously. This is because we, or more like our brain, will make split-second decisions that may lead us to use our off-hand, likely because the action we want to do is better done with that hand (the object is in a left pocket; a cup is on your left side, not your right). The rest of the chapter shows that this character is right hand dominant, so it is a better assumption that those few panels were only really done because of the off-hand exception.
This finding may not completely give reason as to why she used her left hand for the amulet, but I can guess that it's just an author's choice, more specifically "panel flow": panels previous to left-handed actions have Rena in a position where it would better flow with her using her left hand. She approaches Fuutaro from the left side of the panel, so having her use her right hand would display the back of the hand and may not be a good looking panel; when she gets off the boat, the manga goes right-to-left, so maybe Negi-sensei chose to having the panel zoom onto her left hand before she tosses the amulet (also, having her back to the reader is more ominous/cold than if she faced towards us). My guess her may be as good as mine, and neither do I want to discount your efforts of going through the chapters for handiness. But with the above in mind, this left-handedness myth is debunked sadly until we get more Rena screentime. (also, in the panel where she holds the childhood pic, that is a right handed gesture, not left)
What I do find more valuable now is the findings of the gals' habits. Maybe when we get to piece together more, those bits will help us.
@dube There is a difference between well-done realistic characters and poorly done realistic characters. It is true: humans as they are can be selfish; they can be unreadable. However, people can also be selfless and intelligible. And when making a compelling character, a balance needs to be struck, because go one way and a character is too down-to-earth and the other way they require the character to suspend their belief too much. True, reading and other activities are an escape, but not all readers are looking for rose-tinted fantasies. Some want a dip that is a bit fictional; others want to read a story of non-fiction, but cannot partake in the event for time-place reason (wrong locaiton, wrong era/time). As you might know also, reality can be stranger than fiction, as there are people who seem impossibly unreal, either because they are overly obedient or very unreasonable to a fault.
To state also, none of this supports why Ichika is even a complex character. Sure, I can see where realistic may come fromt, but complex has not come across your statement here or before.
The fact that you choose to be condescending by saying "Manga readers struggle to understand X" (even if true) in itself does make your claim less a founded statement and more of a self-righteous comment.