The first two thirds of this episode concludes Renji’s story, though I’m not sure if a miracle actually happens to allow Chihiro to overcome her disability (since her representative chains DO break).
There wasn’t any miracle, but by learning to trust Renji completely, Chihiro can borrow his memory (to an extent she can access it with words — she cannot see what he remembers unless he suddenly develops a talent to sketch). Therefore, she only needs to keep thinking about him and their relationship and then she pretty much does not need a diary anymore. This is why chains broke. However, I certainly hope she will not become too careless, because after all accidents happen, and the diary has a certain useful rigidity. I can see Renji asking Chihiro to “recall” certain events simply because his own recollection is fuzzy.