This is an interesting discussion. I'll add my 2 cents.
In response to the idea of whether there was a plan A and B, I submit that there was only ever one plan as presented by the Father in the premortal council. It could be argued that Satan put forth a plan B but it frankly wasn't much of a plan. The need for a Savior was identified as a critical part of God's plan and so it is reasonable to assume that the fall was also an intended part of the plan. And as we consider the fall in light of the big picture it becomes more clear as to why the fall was intended from the beginning. The fall obviously resulted in many consequences, some negative, some positive.
Negative Consequences include: Death – non glorified body, Injury, sickness, and disease, Thistles, briars, and weeds, Earth not produce in its fullness, Enmity between man and beasts, Satan's temptations, and Separation from God.
Positive Consequences include: Death – glorified body (after Resurrection), Gain mortal bodies to learn with, Able to bear and rear children, Able to gain and learn from experiences, Know good from evil, Become more holy than before, and the possibility of returning to live in God's presence.
But when the Millennium comes, when the earth is returned to it's Edenic state, notice that ALL of the negative consequences of the fall are done away with while ALL of the positive consequences remain. So why the difference between the first Eden and the last? The last Eden can only be enjoyed by those who are holy enough to experience such a state and as Adam and Eve quickly found out, many of their children could not inhabit such a place. A fallen world needed to exist so ALL of God's children could come to earth and experience an earthly life. For those who are righteous, or seek to become so, living in a fallen world merely hastens their progression.