I think this is a great question so I will try to offer some light and round out the answers. I will try to answer two different aspects of the question before finally examining C.S. Lewis' claim about Hell's doors being locked from the inside.
Debt and Punishment
The first aspect of the question is as follows:
If our sin does not incur a debt then Jesus' sacrifice cannot make
atonement for our sin. Put differently: if our sin does not incur an
extrinsic punishment then Jesus cannot make atonement for the debt of
The basic idea here is that the penal substitution theory of the Atonement requires an extrinsic debt and punishment, and this is not present in theories which see sin primarily as a choice or predilection. The person who has abandoned penal substitution because it seems to make God vindictive is thus faced with the question: What was the purpose or value of Christ's passion and death? How does it help us?
The short answer is that the person who opts for a choice/predilection emphasis regarding sin and Hell will need to turn to a different theory of the Atonement than penal substitution. The "Christus Victor" approach is a very common one. It would say that the reason we choose sin is because we are in bondage to sin, and that Christ saves us by breaking the bonds of sin in his passion, death, and resurrection. Once we are freed from this bondage we are able to choose God with the help of grace. See this page for other theories of the Atonement.
The most important part of this question lies in the deep Biblical roots of substitutionary Atonement. It is not clear that the Atonement theories which altogether abandon substitutionary concepts can adequately represent the Biblical testimony.
Choice and Pelagianism
The second aspect of the question is as follows:
If Hell is at bottom just a choice that a person makes, then they
could just as easily change their mind and leave Hell. If they
locked the door then they can unlock it. But this is problematic because
it implies that we can save ourselves.
Most all Christians acknowledge that sin involves choice. If we are forced to do something bad then we usually aren't sinning. We have to actually choose to do something bad before we can be considered guilty.
In Christian doctrine Hell is irreversible. Justifications for this vary. For example, in Catholicism Hell is irreversible because we can no longer make the relevant kinds of choices after death. Our choice in life is our choice for eternity. So the first point is that we can't leave Hell because Hell is irreversible and eternal.
The second point is that sin and repentance are asymmetrical. It is a basic Christian truth that we can do evil by ourselves but we need God's help to do good. So we can sin by ourselves but we need God's help to repent. We can choose to sin on our own, but we need God's help to choose to repent. This is just a basic orientation, at least for Catholics, Orthodox, and Lutherans.
C.S. Lewis and the Doors of Hell
Finally, what did Lewis mean when he said that the doors of Hell are locked from the inside? I think he was trying to say that it is not God's fault that someone is in Hell, but rather that person's fault. It is their own doing, not God's doing. God wanted them to be saved, but they ended up in Hell through their own free choice.
But we must further examine the fact that he seems to be implying that the damned prefer to stay in Hell, for they have locked themselves in. Reading this through works like The Great Divorce, I think the correct interpretation is that the damned, through their own free choice, have departed from God and diminished themselves to such an extent that they prefer to remain apart from God. They have become haters of God. God's light burns them, for they have become so evil that they shrink from his Goodness. So they lock the door to stay far away from God.