So if a perfect picture requires black. Then a perfect plan requires evil and sin? I do not think this is right. God is omnipotent. He can make a perfect plan without evil and sin. He has that power.
Perhaps an issue you are facing is your definition of "omnipotent". Omnipotent does not mean God can do anything. It means He can do anything within His character and values.
For some reason (we can only speculate as to why), it is consistent within God's character to not save everyone.
I think it is an excellent question you ask, and I have asked the same from time to time. I regularly pray that God would save everyone, but I don't think He will, and I really don't understand why. Lately, I am more and more just praying for the people He has chosen to save.
I think you should continue to seek answers to this and like minded questions. Who knows? Perhaps God will lead you to the answer, but unfortunately, it is not the kind of thing an average Christian is likely to have an answer for. And, as shown above, many will not even understand the question clearly.
I think you have been responding to the questions in a mature manner and I commend you for that. I also encourage you to continue searching for these kinds of answers. It is a way we seek the truth of God.
I also want to mention that although God wants us to seek the answers to such questions, it is not the most important thing He wants from us. Most of all, He wants us to love Him back for all the love He has shown to us, and to reflect that love to the world around us. But the more we can answer the kinds of questions you ask, the better we can show His love to certain groups of people, and the more fully we can enjoy and understand God's truth and wisdom.