Honestly, to me the netorase part feels weirder to me then the netorare one. Mostly because the MC does nothing with his girlfriend, even knowing she isn't all innocent.
Also Sayako's comments in Kana's scenes after the 1st village feel more like netorare to me. Less "MC let's me fuck his girlfriend" and more "I am going to fuck and corrupt this girl and MC might watch"
All in all, MC and Kana becoming a couple feels...unnecessary, since they hardly interact afterwards.
I mean the story part, if we talk about character design, it is true that the protagonist is too open-minded not to have had a relationship, the thing would be that at first it was ntr and that the protagonist begins to discover his kink through their party while sayoko has her fun they don't really have a romantic relationship and then to the end he realizes that he likes to see sexual intercourse and can be more open to sharing his girlfriend in the end
Or if we go through a ntr route, the same thing , Yuki is a lesbian so it is impossible to have a relationship with the mc and he can be a great accomplice to corrupt the whole party and in the end as a target for Kana, everything can be compatible
As the game asks you at the beginning, you can take a role of ntr or netorase without creating two parallel stories and it would also help the development of the scenes and also be able to focus on the other characters because all the scenes of kana can be used for a netorase route that for ntr the only thing is to change something in the dialogue to make it neutral
It's how I see it, although it's true that everything felt rushed and very strange, so I understand you.