The question that Alma is answering is how is it just that God punish the wicked if he saves the good. The simple answer to that is the plan of redemption, but Alma cannot explain the justice of that plan without discussing what would happen in its absence. That is the reason that he “lay[s] aside” the plan of redemption. It is laid aside for the argument precisely because it is the answer to the argument. However, the reason that it is the answer is best seen when examining a world without the plan of redemption.
Thus, we have in verse 12 the condition where there was “no means to reclaim men from this fallen state, which man had brought upon himself because of his own disobedience.” Agency virtually assured that we would disobey and therefore merit our fallen state. Without the plan of redemption, and with already having eternal life, we would be condemned forever, as there was no way to separate us from our sins (again, without the plan of redemption).
Without the barrier of death, there was no way to separate humankind from their life in the mortal state. Even repentance would be ineffective as there was no way to implement repentance because there was no barrier of death and subsequent resurrection that would allow for good changes in our behavior to be effective and for repented sins to be forsaken.
Alma concludes that in that state, where humankind was subject to sin, but not to death, they “were in the grasp of justice.” Our actions would inevitably lead us away from God with no way of reconciliation. In those conditions, justice prevailed, and mercy had no ability to act.