It sounds as though you have you have interpreted the above verses as God instructing Cain on how to make his offerings acceptable to God. But if you read them carefully, you should conclude that God wasn't giving Cain instructions but He was rather, giving him a reason why his offering was rejected.
At no point did God give Cain a promise to accept his offerings in the future, if they were offered according to Gods requirements. God was actually saying, if Cains offering was acceptable He would have accepted it.
And sin was lying at the door, because his offering was unacceptable. This meant that Cains sins were not atoned for and sin desired to rule him and he would need to rule over it, to avoid being consumed by it.
This is a clear indication that Cain was not one of Gods elect, because God knows that sin always wins over those who fight it on their own, without God. Cain quickly demonstrated why man can never have victory over the power of sin, unless God fights the battle for us. Cain went on to murder his brother to prove he has no power over sin.
God never gave Cain any indication that He might accept any future offerings, even before he murdered Abel. The fact that he murdered his brother shows that he was not one of Gods elect, and as such he can never become one.