I feel for the OP. What he/she is asking hasn't been answered at all. In my understanding of Islam, one of the greatest strengths of the tradition is being able to say "I don't know." I agree with the OP, God is all knowing & did has DESIGNED each of His creation, meaning how would it think, how would it respond to challenges, how would it behave, etc. As He MADE us. So the question is absolutely valid, but the answer thereof, I have not found in any religion I have studied over the last 12 years.
Brother/Sister OP, I have grappled with this question for many years, i have come to realise that it is from among the matters of the unseen, which we, in this human state/form, have not been made privy to due to the nature of the test we have to undergo here. If everything was so crystal clear, there would be no way to REALLY test our faith/Iman. There has to be paradoxes in our limited view/perception of creation, seeming hypocricies.
Faith, by definition, is relatively blind. However even then, the clear signs of God's existence & power are clear. For eg, we don't need to beat our own hearts. I know your question does not challenge the existence of God, but I fear that this particular doubt may lead there eventually.
God does not do "an atom's weight of injustice to anyone", and personally I feel it may be that on the day of judgement, those actions which God knows a human or jinn has done which was evil but was not entirely its own decision, He may forgive. He is the All-Knowing, He has all the info, we don't.
Now the paradox is that no religion can start teaching ppl this, because then u will get fatalists & determinists who will do evil without struggling against themselves & simply claim they are instruments of divine predestination. Simultaneously, there has to be at least a seeming choice between doing good & evil in this world, between denying or submitting to the One by which we exist, in order for the test of faith, which is the purpose of this realm, to be fulfilled, as God Himself says He has not created this realm, or us as humans, or the jinn, in vain.
This is what I understand of this deeply perplexing riddle thus far, wa Allahu ta'ala Alam, God knows best.
Hope it helps my dear Brother/Sister.