"If God is omniscient and omnipotent why didn't He stop Adam and Eve sinning, and keep the world perfect".
The reason we were created here rather in the presents of God like the angels were is so that we may live a life that is centered around a choice. To either choose to be with God for all of eternity or to be alone with ourselves. This is why the tree of the knowledge of good and evil was placed in the center of the Garden. If God's intent was to create a perfect place He would not have placed the tree in the garden..
But like with any choice there is consequence. For us the consequence of sin in the face of a Holy and righteous God is death. God over comes this natural consequence with the sacrifice of Jesus. This allows us a true choice. We can freely choose to be with God or we can spend eternity away from God.
don't see that omniscience and omnipotence can go entirely hand in hand (because then God is limited by what He knows - if He always knew that Adam and Eve would take the fruit, He could do nothing to stop it, and is therefore not omnipotent).
This whole line of reasoning is predicated on the Idea that the Earth and all that He created that first week was set up to be our final destination.. This Earth, this life, is merely a stepping stone to the door way of the rest of eternity. It is the place where we are to live for a time so that we may be able to choose how we wish to spend the rest of eternity with out the direct knowledge of who God is, and what it is He has to offer.