The only justification for eternal punishment would be eternal sin. Even though you quite clearly mention unrepentant mortal sin, I don’t know that you’re fully thinking through the ramifications of what it means to be eternally unrepentant. It’s not coveting another man’s wife, in and of itself, that damns someone to hell. It’s coveting another man’s wife and never being sorry for it.
God has the power to forgive ANY sin, no matter how heinous. He’s God, after all. But because he respects our free will, God will not forgive any sins we do not want him to forgive. If a sinner eternally refuses to ask for forgiveness and eternally remains without sorrow for their sin, then an eternity in hell seems quite logical to me.
All sin is not punished equally. Only unrepentant mortal sin carries with it the worst punishment. Venial sins do not carry the worst punishment. Mortal sins for which we repent do not carry the worst punishment.
Shoplifting food is not necessarily mortal sin. There are three requirements for a sin to be mortal. First, it must be a sin of grave matter. While stealing is always a sin, if the quantity of food that is stolen is small then it may not be grave matter. In addition to always being a sin of grave matter, mortal sin is always committed with the knowledge that it is wrong. God does not punish those who are ignorant. And finally, the sin must be committed freely without coercion. If coercion is involved, such as threats on the part of another, or the person is impaired due to mental illness or addiction or something else that impedes free will, then they are not fully culpable for what they do. Depending on the extent of the impediment, they may not even be culpable at all.
If a single one of those three requirements is lacking, a sin is venial rather than mortal. And even if a sin is mortal, it only results in the eternal punishment of hell if the sinner never asks God for forgiveness.