I will begin by saying how much I appreciate this in-depth discussion.
I appreciate the effort you have expended in developing your theory, and also I have begun to understand the reason(s) why so much emphasis has been placed on seeing human history and the human conditions through a Darwinian evolutionary matrix.
I need to say that your theory of “God never remove Original Grace from human souls, but doesn’t give Original Grace to the human souls He creates after the first sin in human history” seems to presuppose that:
(a) Knowledge of God was somehow available to all, humans or non-humans.
(b) That without knowing the Law, people could be considered to have sinned.
© That the human condition is adequately understood by us, to enable us to identify a time when sin was absent and after this time, it became part of humanity.
The question of God’s grace, and the redemption of the creation, cannot, in my humble opinion, be discussed within a physical time framework – by that I mean God’s will transcends our experiences and conceptualisation – although we rely on guidance through scripture and pray that we will receive this from the Holy Spirit.
Getting back to the main point, we are given information through scripture on Adam and Eve, and we understand that they did not obey God’s command, but they were deceived into believing error, and consequently acting contrary to God’s command.
Paul discusses knowledge of the law and the importance of deeds, (Rom 2.6 God will render to all according to their deeds) – with Adam and Eve we are clear, and they were removed from Eden, but they could still pray to God (enter Cain and Able).
I am inclined to the view that the relationship was changed, in that before Adam and Eve were deceived, they had unhindered knowledge and understanding of God. After this act (or deed), my speculation differs from your theory, in that Adam and Eve provided an erroneous understanding of God to those outside the garden prepared by God, and sin=error (even before the Law was presented formally) became universal.
I will end by agreeing with you in that God has withheld His Grace from portions of humanity until the time Christ walked amongst us, and this too testifies to His Mercy and Forbearance.