Thank you for writing yet another interesting article with lots of questions that no doubt will bring a robust discussion.
Let be begin by explaining that unfortunately after the Protestant Reformation in the 1500's there have been many so called "Christian concepts of salvation" that have been developed.
I'm Catholic and therefore my comments and answers concerning Christ's Redemption, salvation, forgiveness of sins, etc. will be from over 2,000 years of constant Catholic Church teachings.
I have often read in this blog, as in many others, about the Christian belief that Jesus died for humanity's sins. Frankly speaking, it sounds illogical to me. Really, what does this mean?
It's true Jesus Christ died for humanity's sins and this Christian doctrine is called Christ's Redemption of the human race. Christ died on the Cross and redeemed all mankind.
To properly understand Christ's Redemption we must go to the first Book of the Torah, Genesis.
In Genesis we learn that sin appeared in the world when Adam and Eve disobeyed Almighty God. As a result of their sin, they forfeited the friendship of God, the gates of Heaven were closed and they became enslaved to the Devil.
Then we learn from Genesis that Almighty God is Patient, Gracious and Merciful...before He drove Adam and Eve out of the Garden into the misery of the world, He promised them a Redeemer who would come and overcome death, sin and the devil and the gates of Heavenly paradise would be thrown open again.
Just as all the Old Testament prophets prophesized, the Christ came and by His death on the Cross, in which He became a substitional Victim for the whole human race, paid the price of our redemption,and by His satisfactions, He blotted out our sin that we all inherited from Adam, and by His merits He won for us the restoration to the grace and friendship of Almighty God.
So in a nutshell, the doctrine of Christ's Redemption of the human race comprises the element of an initial loss and a restoration at a price paid by Christ.