Great post, wish I had mod points. I agree that we need to find a well-defined event, one that is measurable and scientifically derived. However, I fear that it will be a very long time until there is consensus. In my mind this problem is reminiscent of, in AI, defining consciousness in scientific or mathematical terms. It may be that we simply don't have the understanding yet to answer it one way or another, and so are left with the blind leading the blind.
Additionally, thank you for pointing out the obvious. In all my thought regarding other key events such as development of beating heart, brain structure, and birth, etc, I never realized that there was another that occurred so early. Probably because I do not consider an embryo a human, and I was mentally lazy. I like undermining my own prejudices with new information and logic. But opinions aside, formation of unique DNA has a lot going for it, logically.