I do want to give an answer to this.
I'm not 100% sure of the underlying logic and if it is fully sound, but I still want to give it a shot.
Also I said I didn't wanna talk about christianity online anymore, so this is an exception, I don't want to debate a lot, jusst giving maybe an idea.
First off, on prayer. God is not a vending machine, never has been. Christianity expressly teaches that no one can do against God's will, and God's will is sovereign in the world. God will answer to your prayers, but that answer can be "no". As Christ himself stated in his teaching on how to pray God : "Thy will be done in heaven as on earth".
Second of all, why evil ?
The first answer is free will, regarding humans. God gave Adam and Eve a choice, for them not to be mere puppets but willful servants, priests by choice. They chose.
But what about other evil, like the suffering of animals, and the unjust suffering of the righteous ? To me this is the biggest problem that is brought up by the problem of evil, and it is also the most mysterious, because in christianity, the biggest message surrounds this problem : Christ.
Christ is the most just, and suffers the most unjust death. He is the living and dying embodiment of the problem of evil. Why does a man as righteous as Jesus suffer unjustly, not just physically but also spiritually, as he is abandonned by God on the cross, while carrying the worst filth that he did not commit ?
The answer to that question can be found int he Acts of the Apostles 3:18, and in numerous other passages of the New Testament : "But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.". In other words, Christ HAD to suffer, it was a necessity for the wellbeing of a plan, and in order for Christ to be born, numerous people had to suffer before him.
One thing to understand about God, is that God usually is described as being outside time. He does not live the present like we do, past present and future are one to him, and they go together hand in hand, you cannot just take a piece of the puzzle out and claim it still works. What happens needs to happen for whatever the future hold to happen.
Another great piece of evidence is found in the Bible in the book of Job, where Job laments his life and doesn't understand why what happens to him happens. He turns towards God, and calls himself righteous and his fate unjust, as God should reward a man as just as him with great goods. Instead, he rots on the ground and still refuses to curse God. He remains just, but sour at his fate and at God.
His friends try to convince him that he surely had done something wrong and evil, and they follow the same logic as him, by saying that surely what happens to him is a punishment, because evil only happens to evil people and good things happen to good people, but Elihu, at the end of the book, the youngest person, and which also means "God himself" in hebbrew finally accuses Job and his friends for their wrongdoings and the wrongness of their speech, by saying that Job accuses God of being wrong, wrong about him and his fate and wrong in judgement. Elihu says that instead of glorifying God, Job turns his back to him as soon as things do not go his way.
And at the end of the book, when God himself finally appears, God asks Job :
Were you there when I made the world?
If you know so much, tell me about it.
Who decided how large it would be?
Who stretched the measuring line over it?
Do you know all the answers?
What holds up the pillars that support the earth?
Who laid the cornerstone of the world?
In the dawn of that day the stars sang together,
and the heavenly beings[a] shouted for joy.
God made the world, knows what needs to happen, and what needs to be avoided in order for things to happen. He orchestrates things in such a way that the outcome he decides of happen, and nothing can be taken away from it.
One may ask : "But couldn't God make things happen without the evil in it ?"
And the answer to that is simply no. We wouldn't be in the world we are today of the black plague didn't happen, if Hitler didn't exist and if natural events didn't occur. We wouldn't exist, and wouldn't be talking on here. Some evil needs to happen for the greater good to exist. And some evil needs to happen for free will to exist.
One might say : "Well doesn't free will contradict God's sovereignty ?"
And to that I admit of having no real answer. I see both as existing, undeniably. I know to be free of my choices, to have a soul able to decide, that I am sure, I know myself able to doubt, able to question and to chose. But I also know that my birthplace influenced me. So I do not know.
This is the one place where I actually use the word "mystery", because I know both exist and coexist in a way that I do not understand.
But if both exist in a mysterious relationship, then I think this can be an answer to the problem of evil