This is a remarkable view!
I think we are basically stating the same with different words. So I put my claims in relationship with yours:
You distinguish between the “population of Image Bearers” and the whole population of (I presume) Homo sapiens (or “anatomically modern humans”). The “population of Image Bearers” went from 0% to 100% over some period of time.
I call Humanity the “population of Image Bearers” and claim Humanity went from 0% to 100% of Homo sapiens over some period of time.
You claim: “By the time the Bible was written the percentage was at 100% and so it can say we were all born in the image of God”.
I fully agree and would like to refer to Genesis 9:6 as biblical support for your claim: It is at this moment that the percentage becomes 100%.
It seems rather obvious that Genesis 1:26-28 and Genesis 2:21-24 can be considered Biblical references to the moment when the percentage ceased to be 0%.
It is also obvious for me that according to these Scripture pericopes the descendants of Image Bearers are Image Bearers as well.
Interestingly, between Genesis 1:26-28 and Genesis 9:6 there is another Biblical reference to population as Image Bearers: Genesis 5:1-2.
This pericope can be interpreted in the sense that the percentage of Image Bearers was increasing because God was creating new Image Bearers, who were not biologically descended from the first Image Bearers (“Adam and Eve”). This fits rather well to the episode in Genesis 6:1-4, which on the one hand can be considered bridging Chapter 5 and the account of Noah and the Flood, and on the other hand refers to “Sons of God”, that is, humans in the Image of God who had no mother or father like Adam. This interpretation is supported by Luke 3:23-38, which includes Genesis 5:3-32 within the genealogy of Jesus-Christ, and uses the term “Son of God” to qualify Adam.
In conclusion: Genesis 6:1-4 appears like a clear reference to the period of time when the “population of Image Bearers” went from 0% to 100%.
I finish by commenting your challenging claim: “Image/Non-Image applies to a population not individuals in the population.”
I think it contains a brilliant insight but should be formulated more accurately as follows:
“Image applies to both a population AND the individuals in the population.”
This relates to my claim that a population of Image Bearers is a population called to live according to moral judgment and the “Golden Rule”. Thus, it would not make sense calling “Image Bearer” a single individual alone without reference to Humanity.
The Incarnation of God reveals us the dignity of the human body, that is, individual and humanity all in one.