Even though the Past arc was a plot tumor, it now makes sense why the author was so intent on prolonging it for so long. Why? Though "Zero" is Hime's future love, the arc itself focuses on the Character Development of Rei—because Rei IS the "Zero" of the future that we know!
The reason Zero very easily became the God Of Death in the present even with the help of the MacGuffin; he had become a Shinigami in the past as Rei, however briefly, hence the darkness in his heart. It's very possible the "kind facade created by an idealistic youth" that "Zero" was talking about was created by Rei in an attempt to live up to who he believed Zero would grow up to be.
The events of Chapter 71. Asura knew from the beginning that she would be a part of the Ultimate Sword, and her trouble transforming back into her normal self is a result of her resentment about being treated as spare parts. Himeko's decision to spare Asura is what proved to Asura that she was wrong about being a mere "substitute" for Yamatohime and the Ultimate Sword; if Himeko hadn't done this, Asura wouldn't have had the time to learn that she was the only one who thought of herself AS spare parts.
Zero REALLY loves his scarf. He considers it the red string of fate between him and his beloved, because his beloved was the one that gave it to him in the first place.
At first, the dissonance between Zero's actual age and his youthful appearance seems to make no sense. Then the Past Arc implies that Rei's hatred and vengeance towards Doc stunted his growth, and Zero's death seemed to push his growth back even further. It is possible that at the beginning of the story, Zero could have been in the middle of his second growth spurt, especially since the first one had been held back for so long. He looks as young as he does because his body's finally getting around to catching up with his chronological age.