I agree with much of what you wrote, and you argue your points persuasively. Don’t worry yourself that I’m not being fully convinced, I’m one of those people who think abortion is an indeterminate issue that can’t be neatly and quickly rationalized by either side.
We agree that the embryo is a living organism of the human species from the moment of conception. You brought up an interesting point about our ‘lifetime’ beginning at conception. We might, in the future, then add 9 months to our ages!
I believe there exists a spiritual soul and a human spirit. The ‘human spirit’ I would call ‘personhood’ – the thing that makes us different than animals. The moment when each of these begins is very unclear and open to speculation, and I think this is what nearly every abortion discussion boils down to.
It seems to me (what presumption!) that God is interested in personhood, not just static life. Personhood is when one would have a chance to make moral or immoral decisions and face ethical dilemmas. Your ‘soul’, I believe, is grown by your conscience, which doesn’t appear until ‘personhood’. Before that, if it exists, it is neutral and untouched. Then the question becomes, is its essence defined by its function or does the soul go through a life cycle – being conceived, birthing, growing, then dying? In short, I don’t think God is interested in the science, I think he’s interested in whether or not our soul has had a chance to begin a relationship with Him.
I’ve not heard the ‘twinning argument’, perhaps you can explain it and your reply to it.
What you really have to ask yourself, is what kind of "person" would risk killing a living soul that the Lord knows?
The next question should be, how do you justify saving the ones (souls) you would save to those who don't believe in the Creator or souls, but who make up a large proportion of the voters and legislators?