The penalty for sin is death. If God could create humans that are free from the burdon of Original Sin, He never would have needed to die in the first place. And His law would mean nothing. But since all humans are born into sin, our only hope for salvation is through His resurrection.
OC: He created exactly 3 humans free from original sin: Adam, Eve, and Mary. He chose to bring his Son, the second person of the Holy Trinity, into time through birth by a woman. He was at once human and divine. That woman, Mary was held free from original and other personal sin from her conception through her natural death and assumption into heaven for the purpose of bearing the Christ, the Messiah within her body. Intellectualizing this will never get us there. We are called to have faith.
The salvation Christ provided would not have been possible if He hadn't been sacrificed. If He had been born of Mary, but died of old age, He would not have fulfilled the law. It was not His birth that saved us, but His sacrifice and resurrection. Therefore, Mary was not THE requirement for the fulfillment of His law. She was one part of it. In addition to Mary, Christ "needed" somebody to sentence him to death. Pilate was just as much a part of that plan as Mary. Both were human, borne of human, sinful parents. Neither were God. I believe only God can be free of sin. I can't find anything in God's inspired Word that says otherwise.
OC: Her yes to God was a requirement in order to fulfill God's plan in the manner to which He wanted it fulfilled. We can speculate all day on other means God could have used for Jesus to enter time, but the fact is he used Mary with her full consent. Nobody else on the planet at the time could have conceived the Messiah. Your assertiion that she wasn't THE requirement is correct, her fidelity and trust in the Lord was INTEGRAL to God's plan for salvation. Pilate on the other hand was in the wrong place a the wrong time. Whoever would have been procurator of Judea at that time would have faced the same situation. He was only integral to the Passion due to his assignment at the time. Had he set Jesus free, the Pharisees and Sanhedrin would have found some other way to kill Jesus. As it was they had waited sometime to do it by the time it happened anyway. But as it happened, Pilate played his part, and had Jesus scourged and crucified.
You are right in that we are all saved by His suffering, death, and resurrection. We are also saved by imitating His life and being Him to others. We have a responsibility to be the body of Christ on earth in our time.
As has already been pointed out, there is absolutely no Scriptural evidence to suggest Mary was born free from original sin. That is no small point. There are several references to Christ being born free of sin. None concerning Mary. There is Catholic Tradition that states that, but given the choice of believing the Word of God or the word of man, there is no choice.
"Pilate on the other hand was in the wrong place a the wrong time."
So Mary was part of God's plan, but Pilate was just a coincidence? Did God not know Pilate's role before he was born? Did God only build half a plan? The Pharisees could not sentence Jesus to death. If they could have, they would have. That is why they brought Him to Pilate. And Pilate could have denied their request. But he played his part. As did Mary. And there is absolutely no Scriptural evidence to suggest otherwise.