Psalm 139 is some very beautiful poetry. However, I don't know of any credible theologian that would be called anything less than sloppy for attempting to take that poetry and use it as the foundation for a theological position such as you are. The more you dig your heels in on this one, the harder it is to actually consider what you say as credible.
Psalm 139:12-17 Even the darkness is not dark to You, And the night is as bright as the day. Darkness and light are alike to You. For You formed my inward parts; You wove me in my mother's womb. I will give thanks to You, for I am fearfully and wonderfully made; Wonderful are Your works, And my soul knows it very well. My frame was not hidden from You, When I was made in secret, And skillfully wrought in the depths of the earth; Your eyes have seen my unformed substance; And in Your book were all written The days that were ordained for me, When as yet there was not one of them. How precious also are Your thoughts to me, O God! How vast is the sum of them!
What a beautiful passage. It really is. Our God is great, our God is creative. For me, this passage is comforting, and leaves me experience the emotion of awe in thinking about how loving and intimately involved with me God is.
But what it doesn't do, and what it shouldn't do, is serve as the platform for some theological argument about when a human being has a soul. It doesn't speak to that, it isn't meant to speak to that. And honestly, it's nothing short of hermeneutical gymnastics to try and twist this passage into saying what you're trying to make it say.
The significance of verse 16, in which the author says "Thine eyes did see my substance, yet being unperfect" (KJV) is as Albert Barnes says, - "Before the embryo had any such form that its future size, shape, or proportions could be marked by the eye of man, it was clearly and distinctly known by God."
The point of the passage is simply to glorify how great God is and that he really knew us deeply and distinctly even before we were born.
Try again.