First, if a blastocyst is a "human being," then the human reproductive system is a deliberately designed holocaust. Second, this mystical nonsense about human "essence" is so utterly undefined (and undefinable) as to be perfectly meaningless.
There is no such thing as a being becoming a human being.
In which case, no line can be drawn, at blastocyts, gametes or prior.
Implantation is a necessary development for future growth of that human being-in-fact, but it has nothing to do with that human being's essence.
I'm afraid you're arguments, relying as they do on this mushy notion of "essence", are entirely arbitrary, and as such, entirely unsatisfactory.
This seems a like negative theological argument in which it is assumed what God would would or would not have done in designing things, and then arguing from the existence of evil. I have thus far not based my argument on theological grounds, but simply on philosophical and scientific grounds.
If the blastocyst is not a human being then what kind of being is it? Is it a radish? Is it a sperm? Is it a chimpanzee?
If the proposition, "I was conceived" make sense, then the ontological status of the blastocyst is self-evident. When did you begin to exist? The beginning of your existence was at the beginning, not at some later time. The problem with your argument is that if you did not exist until you were implanted, there would have been a period of time between the time you were conceived at fertilization and the time you implanted in your mother's womb when you did not exist. This is clearly absurd. Human life is a continuum. Likewise, if you did not begin to exist until you implanted in your mother's womb then what kind of actual being was it that was the product of your parents gametes?
Second, this mystical nonsense about human "essence" is so utterly undefined (and undefinable) as to be perfectly meaningless.
Metaphysics is a branch of philosophy that entails rational thought and the the study of "first principles" and "being" (ontology) There is nothing mystical, arbitrary or mushy about it. Essence refers to the question, "what is it?"
Skepticism about a scientific fact as established as any fact of science, and confirmed beyond cavil that the beginning of a human life occurs at fertilization, is unwarranted. To say that we cannot know what a human being is is also unwarranted. We do not have to know everything to know with certainty what a human being is and how a human being's life begins. To say that we cannot know, itself a claim of knowledge that we cannot know, is self-refuting.
Cordially,