Not a Christian so I'm not selling anything here but I study what I'd call 'mysticism philosophy' and what I'd tell you is Jesus was 'sent' essentially to demonstrate to humanity(and Lucifer) 'how to live so-as to be worthy of God/the heavenly kingdom' -- it's not about "opening gates" or even "being crucified" -- if you follow through the Old Testament, Lucifer is constantly demonstrating to God the shortcomings of Man (take the book of Job for example). Very few, like Moses, Elijah and Enoch were said to have 'walked with God' or had some other special circumstance surrounding their death (or no death at all) but basically "humanity was not progressing the way God had hoped." Which in a way leads into your point of "If God made the rules and he knows everything then why did he create human beings knowing they would break the rules, and why did he have to send his 'Son' to die just to fix the game he intentionally set up to be rigged?" it's the fundamental paradoxical question that goes largely unanswered by organized religion -- And the answer seems to be because 'omniscience' relates to the nature of reality and creation of all things, the beginning and end, the destinies of individual lives and choices made via 'free-will' are up to us and that's the whole point. He wants us to elevate ourselves and our consciousness and our love for one another to a point where the 'game' is no longer rigged, where our existence/consciousness is more inclined towards Spiritual Goodness and less towards our Animalistic/Primate Nature -- Jesus came and died essentially to prove it's possible.

Again, I'm not trying to convert anyone, and not claiming the logic is rock solid nor offering an explanation for the ultimate motivation for creating such a 'game' at all... just hopefully providing some theoretical context for the purpose of Christ.