Em uma entrevista com a JKR:
JKR: James was immensely brave. But the caliber of Lily's bravery was, I think in this instance, higher because she could have saved herself. Now any mother, any normal mother would have done what Lily did. So in that sense her courage too was of an animal quality but she was given time to choose. James wasn't. It's like an intruder entering your house, isn't it? You would instinctively rush them. But if in cold blood you were told, "Get out of the way," you know, what would you do? I mean, I don't think any mother would stand aside from their child. But does that answer it? She did very consciously lay down her life. She had a clear choice -
ES: And James didn't.
JKR: Did he clearly die to try and protect Harry specifically given a clear choice? No. It's a subtle distinction and there's slightly more to it than that but that's most of the answer. -Emphasis mine.
Agora, o que eu tomo dessa citação é que a razão pela qual o feitiço de proteção funcionou, em primeiro lugar, tem algo a ver com o aspecto de "sacrifício". Harry teve que sacrificar-se também na Floresta Proibida:
...If he could only have died like Hedwig, so quickly he would not have known it had happened! Or if he could have launched himself in front of a wand to save someone he loved. . . . He envied even his parents’ deaths now. This cold-blooded walk to his own destruction would require a different kind of bravery. -Extract from an answer given to me in my question. Deathly Hollows chapter 33
Tanto Harry quanto Lily deram proteção aos seus entes queridos, mas sob circunstâncias diferentes. As únicas semelhanças que encontrei entre as duas instâncias é o "sacrifício" que precisava ser feito. Eu acredito que se Lily de fato se afastasse, ela teria sido poupada, já que Voldemort odiava desperdiçar sangue mágico.
Portanto, eu concluo que se Voldemort realmente pretendesse matar Lily, ela nunca teria tido a oportunidade de se "sacrificar" e sua morte teria caído na mesma categoria que a de James, não dando a Harry a proteção.Nota- a perda de sangue mágico referência é uma citação do filme, e desde que eu não tenho acesso aos livros agora eu não tenho certeza se este é livro canon.