No full piece today, but some notes.
In the OT, I suppose the justice of God has to be judged by the covenants to which He binds himself. I'll have to think about whether I think there is any sense of universal justice to God in the OT. Frankly, God is God, El Elyon, the most powerful of the gods and the only legitimate God for Israel to worship. In such a case, justice seems to be God's vindication of Israel on those who dare aggress against her. Also God has certain items like His ark or temple or city that you don't mess with or face God's wrath.
God has also bound Himself to a specific covenant of blessing with Israel. In that sense hesed is His covenant people is blessing them when He said He would and punishing them when they don't keep their end of the deal. There are parts of the OT where punishment seems automatic. Uzzah dies apparently without divine deliberation.
But God can show mercy without anyone having to pay the price. There is no sense of penal substitution. The Day of Atonement goat takes the defilement and pollution of Israel away from them, it doesn't take on the guilt. And, in the end, God desires obedience rather than sacrifice.
In the NT, Jesus is a ransom and a sacrifice, but again, the NT has no sense of Jesus paying any mathematical equivalent of penalty. The Parable of the Prodigal Son indicates that God can forgive us on His own authority. His justice does not have to be satisfied legalistically.
Nevertheless, Jesus' death does show that God is just even though He has passed over previous sins. The desire to show God as just is therefore in the mix.
The question of eternal punishment raises the question of God's justice. Eternal punishment is infinite punishment. Is failure to accept God such a sin that justice requires eternal punishment? Perhaps it does.