I really want him to hook up with the kouhai. As much as I feel for him and his reincarnated wife, he needs to move on. In fact both of them do because clearly we're seeing her original determination to be a new person slipping day by day as she interacts with him. By the time she would even be legally allowed to be his wife again, he's going to literally be a geezer (and as much as I don't like the idea of personal suffering due to societal pressure), everyone is going to question his motives always hovering around a child and especially if they ended up together again. He won't care, but she will and it will probably torture her.
She needs to either lose her old memories again, or form a clean break on her own. I don't think the husband is going to reach this answer on his own, though I'd like to see that possibility.