what tools are available to God to decide when a person is born? Does God have a pool of people available to him that he could create?
Middle knowledge, as you may well know, is simply knowledge of the counterfactuals i.e. what would have happened. The way I understand God’s ordering the world to achieve His purposes is as follows. God’s will is free in that He is free to create or not create a world of creatures with free will, but once God creates a world of creatures endowed with moral worth, moral responsibility and freedom of the will, His purpose with regards to their creation is immutable i.e. His plan for them doesn’t change which is have a nation of people who freely choose to come into an everlasting relationship with Him, the ultimate Good. So you have two immutable constraints to human history i.e. the creation of the world on one end and the divine fulfilment of human history on the other end i.e. God’s purpose for creating humanity. This is why God is called the beginning and the end in that human history proceeds from Him and He is its ultimate fulfilment.
So then what happens in between these two events i.e. the creation of humanity and the fulfilment of human history in God. This is when all the free creaturely decisions occur. But we need to take a step back. Human history consists of two things, free human actions and free divine actions. Middle knowledge dictates that God knows all the possible outcomes bounded by the two constraints i.e. His creation of humanity and the fulfilment of human history in Him. Intertwined with free human decisions are God’s free acts such as commanding Adam, calling Abraham, creating the nation of Israel, the captivity of the Israelites, Jesus coming in the flesh and dying on the cross, the resurrection, sending the Holy Spirit, inspiring the Holy Bible among many others. These acts also form the world that would be actualised.
To answer your question directly, God may have a range of possibilities available to Him based on the two constraints and His free actions which may deliver to Him the set of events that need to occur for His purposes to be fulfilled. If person A being born in 1950 means that person is lost but that person being born in 1951 means they would be saved, prior to the creation of the world, God knows what type of world he needs to create that would generate the necessary events including the billions upon billions of free will decisions that need to be made by all the individuals that predate Person A in order for that individual to be placed within the circumstances that would lead them to freely place their faith in Christ. God not only does this for Person A but does it for every person that will ever exist. Therefore the world we perceive today was created to possess the right attributes for everyone to be placed in the circumstances right for them to freely choose to place their faith in God. However, that is not a guarantee that the person will freely choose Christ. So it goes beyond my example in that God places us in the circumstances He knows are sufficient for us to freely choose Him. Therefore to readjust my example, Person A being born in 1950 may mean they are doomed to condemnation by necessity whereas if being born in 1951 may mean they have sufficient resources to freely believe in God, then God will at the inception of the world tune all its complex parameters so that all the events that occur incorporating human free decisions, lead up to the events that make it possible for people to be born at the right time and place for them to be in sufficient circumstances for them to freely place their faith in Christ. It's almost as if God has rigged the odds in our favour but some still choose to reject God.
Furthermore, this argument assumes God has this ability to place people in different situations. Given this, why didn't God only create people he knew would respond? Why didn't he only take those people out of the pool of people? Or to put it another way: why is it necessary that people die? Is it to create the circumstances in which people who would respond will respond.
If God looked at this very world (let’s call it World 1) prior to creation and decided that remove all the people that reject Him, then a different set of events would occur (let’s call this world 2) and it may mean some people who in world 1 would have accepted Christ may reject Him or are never born in world 2? The question then is why should God disadvantage those that would freely choose to accept Him for the sake of those that would freely choose to reject Him? The flipside may be that if God were to create a world in which those who in world 1 reject Him (let’s call this world 3) they would all still reject Him. Because if any in world 3 would freely choose Him then God would create instead a world 4 in which those who would choose Him in worlds 1 and 3 would choose Him in world 4. In my opinion, this world can be seen as the optimal world. It is possible that there are other worlds that could have achieved God’s purposes just the same but I am not going to lie saying I know why He would create this one instead of the other ones but since He is morally perfect, we can be rest assured that He has not blundered.
To answer your penultimate question, “why is it necessary people die?” It is not necessary that people die. Remember, God did no create human life for it to be extinguished but for people to live with Him forever. We only die now because the world we live in is suffused with death and suffering but this is only temporary and God may allow it because, perhaps, it serves a purposes in cultivating our characters in our quest to be transformed by God from people oriented towards self to be people oriented towards God.
