ok. so: assuming Harry's memory of the night his parents died is correct, what if the crucial difference is that, instead of shrugging Lilly off, Voldemort accepted her bargain...making it a two-person ritual rather than just a one-person ritual...which meant that not only was Harry protected against Voldemort, now the latter had to actively guard Harry's life.

If a mere exchange of promises were sufficient to constitute a magically binding ritual, there would be no need for Unbreakable Oaths.