If it is impossible for God to create a world in which free creatures are incapable of doing evil, how then does one explain the eternal state where free and morally perfect souls dwell?
I know of nothing to suggest that there is a causal relationship that requires humanity to first make sinful choices before God can transform them into free and sinless beings. [Alvin] Plantinga attempts to solve the problem of evil by claiming God cannot make free will beings who are also incapable of sinning. Heaven proves otherwise. It will be populated by free will beings’ incapable of sinning. Whether they were sinners prior to their glorification does not appear to be relevant. If heaven proves that God can create free will beings who are incapable of sin why did he not do so and avoid the whole fallen world filled with evil scenario? I will suggest that the answer has little to nothing to do free will. It’s all about God creating a canvas to display his glory. Apart from evil what would we learn of God’s mercy and wrath? I think the answer to “why is there evil in the world” is best expressed by Paul in Romans 9:22-23:
“What if God, although choosing to show his wrath and make his power known, bore with great patience the objects of his wrath—prepared for destruction? What if he did this to make the riches of his glory known to the objects of his mercy, whom he prepared in advance for glory”
You write: “If heaven proves that God can create free will beings who are incapable of sin why did he not do so and avoid the whole fallen world filled with evil scenario?”
This statement doesn’t seem correct. If we were created capable of sinning as the Bible seems to indicate, then at no time can it be said that we were created incapable of sinning. The fact that our earthly existence is followed by a heavenly one, does not mean that at some point we are suddenly created incapable of sinning (i.e. it cannot be said of something that it was created with a certain property but also without it).
It can be said, though, that those who have entered into a relationship with God while on earth, will become incapable of sinning in heaven. Perhaps it is precisely because we first have to live with a veil of decision-making (deciding for or against a relationship with God) that we could enter freely into a relationship with Him. In other words, an earthly existence during which God’s presence is partially hidden, is the necessary and sufficient condition for anyone to freely accept or reject God’s invitation to be in a relationship with Him. Perhaps it is the very full presence of God, such as He reveals in heaven, that would hinder people from freely entering in a relationship with Him, much like a piece of metal would be drawn irresistibly to a powerful magnet. Perhaps it is therefore only because a person has freely entered such a relationship with God while on earth, and who then experiences God’s full presence in heaven, that he/she becomes incapable of sinning.
I also reject the idea that the only way for a perfectly good God to make his glory known is through creating evil. The only reason God reveals His mercy and wrath, is because there are creatures like us who have rebelled against God. But such mercy and wrath are functions of His perfect goodness and holiness, it doesn’t define it. For instance, why think that unfallen creatures cannot and would not know God’s goodness and holiness? Why would such knowledge be dependent on knowing his mercy and wrath if these are mere functions of his goodness and holiness in relation to fallen creatures?