Alma makes an interesting tie to his last discussion of the time between death and resurrection. He moves that imagery from the future to the past and applies it to Adam. Just as there is a time between death and resurrection, there is a time between Adam’s expulsion and death. Alma is suggesting that the interval time is important. In this case, it is important because it gives room for repentance. The logic of Alma’s statement is that without that space, there could be no repentance. How could that be true?
The logic comes in the understanding of living forever. If Adam had eaten again of the Tree of Life he would have lived forever. This is part of the definition of resurrection, part of the eternal promise. However, if he had done so his earthly state could not have changed. For Alma, the change in state from mortal to immortal is the symbol of repentance as a change of mental/emotional state. Had Adam partaken of the fruit of the Tree of Life it would somehow have short circuited the plan of redemption, made void the need for a Savior, and Adam would have been immortal, but without an Atonement that could remove sin just as the mortal body is removed in death.