Seems like Reira lost whatever little sense she had in her head after Ren's death. I also suspected little Ren as Reira's and Takumi's, and I still have that suspicion. Why would little Ren love spending more time in London, playing guitar for Reira than being with his sister and mother? Here's my theory:
Reira went nuts after Ren's death, was pregnant [thanks Takumi or possibly Shin...], gave birth but she is nuts. Including little Ren, who knows this is his mother, just look at the way he gazes at her http://www.onemanga.com/Nana/70/21/
Because of this, little Ren was raised with Sa-chan, as Hachi's own. Takumi is a selfish bastard. He could have let Hachi be happy with Nobu knowing full well that in the end, he would chose Reira above everyone else. That's why he is in London with her instead of with Hachi. If they do divorce, I hope Nobu and Hachi happens.
Poor Nana though :ccc