If Voldemort made a bargain with Lily (to spare Harry) and reneges on it, that could trigger a different ancient magic (lying under oath or something). Problem is, that would put Severus out of the picture - no need, story-wise, for him to bargain to save Lily if she's the one making bargains on Harry's life.
I don't agree. I think you still need Severus in the picture to (1) suggest the bargain in the first place, and (2) beg for Lily's life to the extent that Voldemort even lets her talk. Otherwise, why wouldn't he just kill her in the first place, without letting her open her mouth? Notice that, in my "missing scene", the first thing Voldemort does is to ask Lily to stand aside. The thing that puzzles me about the scene as Rowling wrote it is that she apparently does! Why? This rewrite attempts to answer that question.
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I don't agree. I think you still need Severus in the picture to (1) suggest the bargain in the first place, and (2) beg for Lily's life to the extent that Voldemort even lets her talk. Otherwise, why wouldn't he just kill her in the first place, without letting her open her mouth? Notice that, in my "missing scene", the first thing Voldemort does is to ask Lily to stand aside. The thing that puzzles me about the scene as Rowling wrote it is that she apparently does! Why? This rewrite attempts to answer that question.