I think we understand justice differently. Having one stand in place of my punishment is not justice. For example if I murder someone and my innocent brother steps in to take the punishment, that is not justice. This is because I am the guilty party. I am still guilty and our since of justice is not satisfied. The victim's family would not see that as justice. This idea of Christ taking our punishment is a medieval understanding of justice that is outdated. Thus mercy cannot fit into this model.
Justice in the Hebrew since of the word is related to Charity. (This I feel is what Alma is meaning since he is essentially Hebrew.) In the covenant language, God's justice would be for him to full fill his promises to us which is to give eternal life to those he covenants with. Thus in the Hebrew since of justice, the only way for God's justice to be satisfied is to give you the blessings that he promised. Thus in order to give you justice, you must be given mercy I.e. Forgiveness. Thus by giving you mercy God's justice is satisfied because he can give you eternal life. It is not satisfied by a form of payment or punishment.
I also feel that when we enter atonement, we enter a relationship with God. We become one with him. That is his justice or blessing. Thus we will share in his glory and he will share in what we have to bring which is pain and sin. Thus it is painful for Christ to enter into a covenant relationship or atonement with us, because he gets nothing but pain and sin, while we get from him sanctification. Thus mercy from him to forgive us and bear this burden of sin which we bring with us what allows for his justice in the Hebrew since of the word to be full filled. This is the only way Christ will come to know us. By feeling and understanding our pain, sorrow, grief and anger, and sin. Through this process we become one, he shares in my life and I in his.
On another level, God does not demand punishment, it is we who demand the punishment of others for there wrong doings. Thus when we feel the forgiveness of God in our lives we are compelled to forgive others thus mercy satisfies our own need for justice that we hold over others for their sins and offenses toward us. Thus in this aspect the atonement is a communal idea. In order be one with God, we have to be one with each other, so the atonement must address the demands of justice that we hold over each other. To me this is the struggle we have,not having God forgive us, but us forgiving others and seeking their forgiveness.
Thus pain and suffering as described in D7C 19 is what happens if we do not repent and enter into atonement with Christ. There is no payment or set punishment. Rather the pain is the pain that we all feel and will experience if we do not allow him to take it from us. That pain is alienation from God, that he wants to take from us. It is not a fixed punishment that we can suffer and then once it is paid then justice is met. It is there to provoke us to repentance, as Alma the younger tells us. That pain is to turn us to him.