So going back to my original question, you believe that God will impute sin onto the believer if he does not confess his sin to God and take steps to make sure he stop committing that particular sin. Is that a yes?
Incidentally, Romans 5:12-19 is not a difficult context to understand what vs 13 is saying. The entire passage talks about sin as a noun and not a verb. It was Adam's disobedience that caused all of us to be separated from God. It is the sinful nature that we inherited and not the individual sins we committed.
Once you understand this, you will understand that it was Christ's obedience that redeemed us back to God. So, in my belief, saying that our sins,
after we are saved, can separate us from God is wrong doctrine.
Note, this does not mean I am saying that you can therefore go ahead and sin.