Chinami been doing it earlier. Naruse meet again with Fumiya because her husband moved to the same city as Chinami to continue the affair.
Fumiya never actively seek out for Naruse, so he only begin to have affair after he met with Naruse by coincidence. Naruse also never knew Chinami is Fumiya’s wife (but Chinami already knew she’s Takahiro’s wife).
As for the kid... really, parents and kids should take the person who actually raise them up and don’t bother too much about the biological parent. Isn’t that the same thing as adoption?
That kid is dating Naruse’s daughter isn’t he? ^^;;
I like your optimistic view of moving forward for the kids but even then, I think author inserted some good evidence on whose children which.
Well we all know Chinami is a slut at Ch2.. But because of this, it also somehow confirms that Fumiya's chance of his kid is not his, on both girls.
Why?
== 1st Chance - The Swap ==
Chinami - Last chapter confirms she fucks Takahiro on Day 1 of cohabitation, which act of 'sacrifice' but likes it along the way and shown us the changes ( Ch. 3 ). Considering their genetic compatibility, has high chance of impregnation.
Naruse - Takahiro has a chance to fuck her while Fumiya is with Chinami, as confirmed in Ch4( or Ch5? ) by Naruse herself during respective spouse visitation, while Fumiya did only does minimal, which interrupted by the police.
== 2nd Chance - Post Swap ==
Chinami - Last chapter confirms as well that after the cohabitation, the police rescue, she implied she is seeing Takahiro after, even before she and Fumiya move to another city, since she said Takahiro follows Chinami when they move, which can only be true if they still in contact with each other. True Fumiya fucks her, but remember we have the genetic compatibility in play, so Takahiro is fucking Chinami all the way the series. But this is based on the Mexican translation, with my limited Spanish knowledge and DeepL / Google Translate, so I might be wrong
Naruse - since Fumiya has not contacted her after the cohabitation and only has chance to reconnect after the house meet. Takahiro fucks Naruse at this time period.
== then Time Skip of 4 years later ==
Another enforcing clue : The kids likes each other because they are half siblings. Also if you think about it, the kids is already meeting before Fumiya and Naruse meet before, since it is very possible that when Chinami and Takahiro meet, they bring the kids as well ( Chinami knows the convinience store which probably where they meet regularly )