Egg and sperm cells that exist independently within the same female reproductive tract are still haploid. Their mere coexistence in the female reproductive tract does not guarantee they are viable or capable of even participating in a fertilization reaction. I disagree with you here. There are, of course, no guarantees that that zygote or cleavage-stage embryo will avoid spontaneous termination of development, but the mere possibility of spontaneous termination is irrelevant to our definition of human life.
Spontaneous termination can occur at any time during the course of embryonic or fetal development.
I think there is an important distinction between spontaneous termination and termination due to some type of human intervention. According to this presupposition, any mechanical or chemical intervention which prevents the process of cleavage or implantation of the blastocyst in the uterus could be regarded as murder. Any interventions following implantation would also have to be regarded as murder. A blastocyst is different than just any old mass of cells because it has the genetic potential and necessary totipotency to at least initiate the process that culminates in the formation of a fully developed human being, as long as no outside interference occurs. I would maintain that until we get to glory, we will never know the precise answers to these questions.