Your confusing what happens in time verses what happens logical in the mind of God in eternity.
What happens in time is Adam sins by permission through free will, however, the salvation that occurs after the fall is due to God's purpose of election "to" salvation in view of permission of the fall. Hence, in the eternal purpose of God it is permission of the fall, election of a great number of the fallen unto salvation. Whereas what we see in time is the fall followed by salvation. It is the eternal purpose of God that explains salvation in relationship to the fall.
Hence, Adam was permitted to fall by his own free will into sin. God had previously elected Adam to salvation and so in time salvation occurred purposely in the life of Adam due to God's elective purpose.
However, in the case of Cain, he was born with a sinful nature that took part with Adam in the fall as all of the human nature was "in Adam" when he freely chose to sin. Cain sinned in Adam as much as you did and as much as Levi paid tithes in Abraham to Melchezidek. Adam acted as the representative of the human race in his testing of free will. Consider Adam like unto a book and every individual human a word on a page in that book. When Adam acted so did the entire human race as the entire race was genetically "in Adam." Hence, by one man's act of disobedience MANY were made sinners. Getting back to Cain. God did not choose to elect Cain "to" salvation but rather permitted him to continue in the free choices he made to resist and reject God. Cain was born in sin just as Abel but God chose to allow Cain to follow the dictates of his own depraved will while having elected Abel to salvation.
HP: But then you have just blown away your argument of unconditional election. If I am reading you correctly, you must be a sinner to be elected to salvation, correct? That would make being a sinner the condition of salvation, therefore if God is going to save Adam and Eve as He elected, God would also have to elect their sinful state.
Are you certain you want to stay with your stated position?