"What does the creation of Adam and Eve and the birth of Cain tell us about the Godhead?"
With respect to being God's image, not much.
To understand why we must understand what God has in mind when he described making man in his image.
26 Then God said, “Let us make man in our image, after our likeness.
And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.”
27 So God created man in his own image... -Genesis 1:26-27
Sandwiched between God's expressed intent to make man in God's image and the statement that he has so created man is his image is a functional description expressed in a command to humanity or in a conveyance of God's will and purpose for humanity. The functional description of God's image is that they are so made that they will have dominion or otherwise rule over all of God's creation in the earth and sea and heavens. This functional description, expressed as a command or a conveyance of God's will and purpose for humanity to have dominion or otherwise to rule over all of God's creation in the earth, sea, and heavens, explains what God had in mind in making man in God's image.
What God had in mind was for us humans to image God by reflecting His rule and dominion over all of His creation through humanity's rule and dominion over all of God's creation. We image God by our role as rulers. To be an image of God is to be God's representative ruler over creation. We are made to rule on His behalf. This is our primary purpose and function as created beings. We are designated as a biological class of representative rulers on God's behalf. Every aspect of our being is designed so that we may faithfully carry out the God-given duties of the sacred office we have from birth over this creation as members of this creation.
Because God's designation of us as images is a functional role based designation, there is no need to assume that any and every particular feature of our biology or our experience as human beings is itself a reflection of some feature of God what he necessarily has or experiences beyond what He explicitly reveals that He has in common with humanity.
Because God's functional classification of us as his image is built into our biology, all members of the class of humans beings are by nature images of God from birth even if at various stages of our lives we are incapable of fully caring out our created function as representative rulers reflecting God's dominion. We are, at various times, immature images or broken images, or images in rebellion against the God in whose image we are created. We are, however, images non the less as Gen 9:6 makes clear.
With respect to Adam and Eve's creation and Cain and Abel's procreation, these are just practical solutions God has implemented to populate creation with enough of God's images so that we may carry out our function over all of God's creation to rule over all of God's creation - God creates people and people pro-create people. Ultimately we must multiply and fill the Earth to practically further God's will that we actually exercise dominion over God's Earth. The reality of our creation is such that we cannot have dominion over all of the Earth if there are not enough of us over all of the Earth.
While our creation and our own procreative abilities don't say much more about the inner workings of what you may describe as the Godhead, it does express God's character in how He has provided for us, with His power, the means by which we may carry out His will by creating us and, in creating us, giving us the miraculous power to procreate ourselves. From this and from many other examples we may understand God's character to provide for us in that which we may need in order that we may do His will.