Its not about believability, very few people game to capture reality, we game to live fantasies. Its about playing a game for enjoyment not getting endlessly bitched at. There is no enjoyable character interaction, even the flashbacks are not enjoyable as its him been the asshole. Thankfully it does tone down as rebuilding the relationship but still far from an enjoyable fantasy.
That's your subjective opinion, though. I quite enjoy games that take their time to set things up properly and have the characters act in a logical and consistent manner. I like the writing so far, slowly uncovering the mystery of what happened during that missing month, the way the characters try to deal with the situation and discovering how BBBen plans to make the female characters accept sharing the protagonist. There's an actual plot here that isn't solely focused on the sex, a mystery to uncover and personal drama where people aren't acting like complete retards, and the narrative is all the better for it, the story starts strong and manages to carry the momentum throughout.
EDIT: Not to say that stories need to be realistic (this is a harem game after all). You're correct in saying that narratives are only beholden to their internal logic, but the author also needs to take into account the setting his story takes place in. As an example, having a character throw a fireball in a fantasy setting doesn't really require much explanation beyond mentioning that magic exists, while a character performing such a feat in a more down-to-earth story would require the author to explain why and how a normal person can spontaneously generate fire in the palm of their hand.
I like harem stories very much, they're my main fetish basically, but nothing about them puts me off nearly as fast as the author jumping the shark and going for instant gratification, with women immediately throwing themselves at the protagonist and offering nothing but token resistance (if even that) at him having more than one paramour. The greater the reward, the harder it should be for the protagonist to reach it, and the more satisfactory it will be for the audience when it actually happens.
Basically, more than the power fantasy of one guy sleeping with multiple hot women, I am more interested in the interpersonal relationships and how such a union could possibly work without resorting to magic, mind control, coercion, etc., and this game seems to want the protagonist to actually work for his happy ending. Not to say harem stories with more fanciful ways to justify the polygamy are bad by default, but I just think it's more satisfactory to see real (or as real as you can get when writing such a story anyway) characters try to work things out.
As for the protagonist acting like an asshole, that's the point I think. He was not himself, and why he was not himself, why he was acting like he did, or how he was able to amass a fortune and bag three (likely more) hot women is one of the two narrative pillars of the story (the second being avoiding a boat ride School Days style).
As for Nobaku, considering the level of personality shift its inconceivable she did not notice especially since she became his sex slave. Even knowing he had serious accident and knowing he had massive personality shift she went along with it.
We don't really know how he acted around her, or any of the other women either. The most substantial flashback so far, the casino one, had the MC behave like an automaton, one that was apparently able to exactly predict how to bet his money to maximize his winnings.
Also, I disagree with her being his sex slave. Her kink of choice is obviously being a submissive, but it's unfair to say that people that choose such a lifestyle lack agency, since it's their continued consent that allows a dom/sub relationship to exist. She's not absolving herself of responsibility, merely trying to showcase the trust she has towards the protagonist the strongest way she can, which makes his "betrayal" hurt all the more, and her forgiveness make her out as a very good and loving person at heart.
Either this is all a dream or Shizu was watching him even before first accident, is not a coincidence she was on train before accident.
It could just be her usual route to her home/college/dormitory. The station is where he also rescued Emi, who surely was not stalking him. Then again, considering Shizuko's personality, she might have taken a fancy to the MC without him actually noticing until his personality switch.
With Emi even I love the writing and the concept. Though Nokabu is wrong, she is not after the attention, she gets that everywhere, she has come to despise attention and it was his cold indifference and her inability to dominate him that drew her in. You also see from her responses to when he starts treating her as supermodel she gets more upset (talk about poster or lavish praise for her good looks). The more he just treats her as normal person the less hostile she gets.
I do like your interpretation, seems pretty insightful. My point was that she keeps acting hostile with the protagonist but there is really no bite to it. Even after his philandering was discovered she is still willing to try and salvage the relationship, as is Nobuko.
tl;dr - the author is taking things slow, and for that to work the protagonist needs to suffer for a bit before he gets his bearings. He's trying to be a decent person and not hurt anyone, or become a victim of a murder/suicide due to excess jealously. As for the events not being interesting, that's something I can't argue with you against since that is your subjective opinion. I can only say that I like the plot hook and the current pace of plot progression.
What the game does need is a better interface. Navigating the apartment is a chore - there should just be a list of rooms you can go to, or a map like in Legacy and, maybe, also buttons to select what activities to do (train eye/hand/mouth, go on dates, etc.).
If you've saved in the middle of an actual piece of story scene or a wake up scene or something like that it's fine, though.
