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.
With some thought, 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 being quietly resentful of the fact that she was being treated like a tool. If Himeko had not spared Asura, she would not have realized that the only person who believed she was a tool was her.
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.
