First, it's important to note that Jesus did not change the moral law. If divorce was wrong in the OT, it is wrong in the NT.
Second, if a means is inherently evil, then God wouldn't use it as a means. God is perfect. We can argue about first and second causes, God's sovereignty, and all of that, but these things are irrelevant to the conversation. God would not use or condone actions that are immoral. In Matthew 19, Jesus is very clear that divorce was permitted civically in the OT. Civic permission is different than endorsement or use. God seems to endorse violence in the OT, so it can't be something that is immoral in and of itself.
While violence is not in and of itself immoral, it seems that it is only moral in extremely specific situations. It is moral as a tool for the judgment of God. God made humanity and has a right over their lives. He is their creator. It is at his discretion, then, that men and women live and die. In the OT, God brought this violence about by his people, Israel. But in the NT we see that this violence was placed upon Christ on the cross. While we will be judged, this will be a future event and it will be done by Jesus himself. Jesus has likewise called his followers to love their enemies, not judge, and leave vengeance to God. While violence towards others is not inherently immoral, it is only made moral in extremely limited circumstances - when God himself places his approval upon it. It seems like the NT gives some pretty sweeping claims about God's permission for Christians to use violence. This is not a picture of God using immoral means in the OT and rescinding that. It's not an acknowledgement that if God "gave in" to the fall and used a fallen means of violence, then it must still be necessary to use violence now. Those things are not at all true. Instead, the violence God exacted as judgment in the OT has now been tempered both by the violence done to Christ, but also by the example of Christ in his endurance of violence and injustice for even his enemies.