If it were the case that God would compensate people for the evil they experienced while on earth, (and reward them eternally on top of that), then indeed the problem of evil might be solved. Of course, we would still ask why God didn’t just reward people without requiring them to suffer first, but as you point out, an eternal reward makes temporary sufferings look like a rounding error.
Why is compensation the only alternative?
Is it not possible that God could – using pain and suffering – carve out of shallow beings a far deeper being capable of experiencing existence at a far deeper and more profound way?
It may not be a question of trading pleasures for pains until one is fully compensated for pains suffered, by some corresponding measure of frivolous pleasures, until the balance sheet has been closed.
Perhaps what God is doing is using the suffering and joys experienced in this life to carve into every human person a profound and deep capacity to exist at undreamed-of levels.
The assumption you make is that pain or suffering are themselves evil. I am not clear that that is anywhere near the complete story. I would say what is more evil is shutting down the capacity to live fully.
Now, it might be true that suffering abuse can do that to a person, but on the other hand if the person is not merely left to their own resources but is in the hands of God who can use the experience to carve profoundly into the heart of the person in such a way as to make them far more capable of living at a profound level, then the abuse has not caused an evil outcome with respect to the victim, although it had the potential to.
The abuse has, however, been the result of evil gripping the abuser by not permitting him to see the depth and beauty of the life and person of the one abused. Now, again, it may be that God could use the abusive act to transform the abuser by the experience of causing suffering such that the abuser begins to be confronted at a deeper level by the good of the other in light of the darkness of his act. Again, no guarantee, but God can potentially use the act to help the abuser recognize the good at a deeper level as a result of seeing it in contrast to the evil he has perpetrated.
Once again, the paradigm of trading goods may not be an adequate one to depict what is going on. Rather it is more like a painter using light and dark to create a painting (the individual person or soul) or a sculptor using pain and pleasure to carve into the human soul the meaningful topography which will define its uniqueness.
This would mean that real evil or good, in the end, is not the achievement of some overall balance of pleasure over pain, as hedonists or utilitarians would have us believe, but that human persons never achieve the depth of being that we are designed for. Love would be the motive and courage the means by which we transform ourselves under the guidance of God’s grace into profound beings worthy of eternal existence. Not being so transformed but continuing to live shallow or unfulfilled lives could, more properly, be called “evil.”