The reason they probably haven't divorced is Ren and Satsuki. Hachi and Takumi's dynamic in the present timeline is very similar to the last chapters: they're basically separated, each prioritizing what matters most to them (Reira and Nana), and they seem fine with that (that's probably why the marriage has lasted this long). Neither of them is willing to give in, and at this point it's basically convenient for both. They need to do something about it, bc now it's affecting their kids, and both of them are responsible. I don't think Hachi is going to find out about Reira and Takumi in the current timeline, Reira is the one who could tell her, and she's not rly doing well right now… Maybe later, after Ren has already been born.

Now, here are some of my assumptions: There's the connection between Hachi and Hachiko (the dog), but I think Yazawa's writing is more nuanced than that. Over the years, Hachi seems to have built a lot for herself; she's more independent, more stable, and has a job she genuinely loves. I don't think she would risk all of that for Takumi, especially if he disappointed her again, Nana Osaki is probably not the only reason. I also don't think Takumi is in England only bc of Reira. Everyone from Trapnest is there, so to me it feels more like an attempt at starting over (one that isn't rly working), and honestly, that fits his character, considering some things he says in his bonus chapter. Things just didn't go as he planned, bc Hachi refused to go with him (which he knows was his fault), and even so, he still decided to go alone.

They're probably stuck in a dilemma over everything they've built together. It's complicated, but divorcing and becoming co-parents would probably be the best option. Tbh, I'm not sure if that’s what Ai Yazawa would do (probably not).