I think the problem is not focusing on the place where you want to apply the subjective measure. (Again subjective doesn't mean "bad" it just means that the word changes meaning with the subject.)We have been talking about personhood. Dixie is just stuck on the red herring that you employ.
Science clearly impacts the discussion. The problem is confusing a scientific definition as science or as Dixie does a scientific fact and dropping the context.
In science, the definition is only useful to test whether various things meet what the scientist mean by life. Sometimes something fails or passes a test unexpectedly and instead of simply saying it is alive or not, the scientific community changes the definition.
Saying the zygote or brain dead are alive poses no problem in biology. It does not upset the biological definition. It does pose a problem in the legal and medical fields and upsets the definition. Our realization of the heart's insignificance upset the definition.
It is abundantly clear that the brain is the organ of concern in assigning rights and therefore in forming any legal definition of life (if none us had a brain we would hardly care who killed who). We even limit rights of certain people with diminished mental capacity or who's brain has not yet fully developed.
Note to Dixie and the dumbasses: no one says people with less mental capacity are less deserving of medical care. So, no I am not arguing we should be able to kill 5-year-olds or any of you idiots, nor should any appear in the next episode of "Man vs. Wild."
If you ever want the discussion to move past the zygote stage into the embryo stage you need to progress it yourself. You also need to talk about the definitions you want to use. It is equally scientific to talk about "brain death" being the point of death as it is to speak about conception being the beginning of life. The matter isn't suddenly "unscientific" unless you try to say that the embryo isn't "alive" without applying your more subjective point.
Just stick to the point and you will have more success actually having the conversation you want. I surmise that you are having the conversation you want, or you would have done this by now.