His potential plan of redemption became a certain plan after the Fall (Gen. 3). It was not actualized until the first century.
Since the incarnation, death, and resurrection of Christ are under God's sovereign control apart from human contingencies, God did know the redemptive work. He knew it as possible in eternity past, certain in Gen. 3, and actual in about 29 A.D.
God predestined that if the plan became necessary, He would implement it. If there was no fall (unlikely), then the plan would remain potential (God knows reality as it is).
Mormons and Calvinists are wrong to thing that the Fall was necessary or desirable for a higher good.