For numerous reasons, I have only now gotten around to answering questions asked here. I'm not sure that I'm actually arrived at the best explanation, but here it goes.
What is metaphorical is not the condition of suffering and vulnerability to harm and death through evil acts. That vulnerability to harm and death from evil is certainly real. Jesus's suffering was certainly real.
What is metaphorical is that this suffering is a punishment for sins. It is metaphorical because true punishment is directly imposed by a just judge in response to actual sin. Suffering evil is unjust by definition, because no one is ever justified in inflicting evil on another. So this situation of being vulnerable to harm and suffering by evil cannot itself be punishment, but it can be analogized to punishment in the sense of being allowed (though not directly willed) by God in response to the sin of Adam. It is exactly the same figure used when men in the Old Testament are made agents of God's judgments. No one using the power of reason could dream that God places an evil desire in a man's heart in order to punish another man. That would make God a most ridiculous pagan fancy, "personal" in exactly the sort of stupidly anthropomorphic way one might associate with Zeus or Odin. Rather, what this reflects is that God's negative providence allows even evil to have power on account of some good, in that those who suffer evil might on that account be impelled to restore their relationship with God (to the extent it has been made available to them to do so, anyway, through obedience to His will). That is the only sort of good that can come in a world that is fundamentally out of touch with God's will.
Original sin is this condition of one's suffering of evil serving God only by accident, this imperfect sort of punishment in which humanity fills God's will only negatively and accidentally. Even natural good is not consciously directed to its ultimate end. Even the righteous only serve God in this sort of accidental, opportunistic way, limited to the happenstance of God's provision of some or another gift in history. There is no principle by which man can relate to God, just these occasional acts of love that find resonance in the hearts of righteous men. The power of evil dominates, serving God's will only in the imperfect way appropriate to it.
This is the case until Jesus. Jesus fundamentally reverses this order and transforms suffering into a path to God. In this way, all of creation, even evil itself, is subjugated to God's power, even beyond the ordinary and accidental way that all creation is under God's Providence. Thus, all creation is reconciled to God in Christ Jesus; all creation serves God. Indeed, in the end, even those who reject Christ will be reconciled in Christ to God (Col. 1:19-20), but because they do not accept the reconciliation willingly, it will only be experienced aversively as the judgment of God against their actual sins (1 Cor. 15:22-24). But even for those reconciled, it does not end there, because for those who experience this reconciliation, Jesus's suffering is also the path toward actually drawing near to God for those who will walk it. Effectively, Jesus through His life restores the relationship that God had with Adam, allowing humanity to walk through conformity to His life to God (or, like Adam, to reject it). Thus does Jesus fulfill the promise to Abraham. Natural effort (work) cannot bring man back to God, but only trust and faith in the one who reconciles God to man and provides the path to righteousness before Him: Jesus Christ (Romans 4).
Let's return, then, to Romans 5
 Therefore, since we are justified by faith, we have peace with God through our Lord Jesus Christ.  Through him we have obtained access to this grace in which we stand, and we rejoice in our hope of sharing the glory of God.  More than that, we rejoice in our sufferings, knowing that suffering produces endurance,  and endurance produces character, and character produces hope,  and hope does not disappoint us, because God's love has been poured into our hearts through the Holy Spirit which has been given to us.
Thus, the peace Christ has created with God is also the path to approach Him and to share His glory.
 While we were still weak, at the right time Christ died for the ungodly.  Why, one will hardly die for a righteous man -- though perhaps for a good man one will dare even to die.  But God shows his love for us in that while we were yet sinners Christ died for us.  Since, therefore, we are now justified by his blood, much more shall we be saved by him from the wrath of God.  For if while we were enemies we were reconciled to God by the death of his Son, much more, now that we are reconciled, shall we be saved by his life.  Not only so, but we also rejoice in God through our Lord Jesus Christ, through whom we have now received our reconciliation.
Note here the commendation of Christ's mercy and His willingness to die for the reconciliation of all creation from the state of original sin ("while we [humanity] were yet sinners [in the analogous sense, i.e., original sin, and the actual sense] Christ died for us"). Note also that even for those reconciled, reconciliation is merely the first step, as it still remains to be saved from His wrath in the end ("much more shall we be saved by Him from the wrath of God"). Finally, Christ's life is the path by which this additional relationship with God takes place ("much more, now that we are reconciled, shall we be saved by His life"). Christ dies to reconcile all creation in Himself, but for those who also willingly follow the path of reconciliation, He is the path to God, the path to avoid sin and to save people from God's future wrath. This provides the context for the contrast below.
 Therefore as sin came into the world through one man and death through sin, and so death spread to all men because all men sinned --  sin indeed was in the world before the law was given, but sin is not counted where there is no law.  Yet death reigned from Adam to Moses, even over those whose sins were not like the transgression of Adam, who was a type of the one who was to come.  But the free gift is not like the trespass. For if many died through one man's trespass, much more have the grace of God and the free gift in the grace of that one man Jesus Christ abounded for many.
What is important to recognize here is the denial that death is the result of actual sin. That is the import of v. 14; all were punished with death even if they did not sin after the semblance of Adam (for example, infants incapable of voluntary sin). This establishes death as being a punishment for sin only analogously. It also establishes that these analogous sorts of "sins" are being included in the statement that "all sinned" in v. 12. This probably is more clear in the patristic reading of the passage, equally valid from a grammatical perspective: "because death spread to all men, all men sinned." The involvement of all men in sin, in this state of being subject to death and harm by evil, makes all men sinners in a metaphorical sense. Likewise, the universal vulnerability of all men to this condition brands them as (metaphorical) sinners, even those who have no sin counted under the law, even those who have not sinned in the likeness of Adam. Again, death is not literal punishment for sin here, but the power of sin granted to death is LIKE the power of punishment hanging over the sinner's acts. Christ's reversal of this circumstance is quite real, and indeed, it is by contrast with this metaphor that His greatness is proclaimed. What Christ does is far greater than merely saving people from the power of death, but opening up the path of Life Eternal.
 And the free gift is not like the effect of that one man's sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brings justification.  If, because of one man's trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ.  Then as one man's trespass led to condemnation for all men, so one man's act of righteousness leads to acquittal and life for all men.  For as by one man's disobedience many were made sinners, so by one man's obedience many will be made righteous.  Law came in, to increase the trespass; but where sin increased, grace abounded all the more,  so that, as sin reigned in death, grace also might reign through righteousness to eternal life through Jesus Christ our Lord.
Note the references here to the reign of death. One man's sin causes the reign of death, condemnation for all men, and so does one man's life (His act of righteousness) lead to acquittal and life for all men. Likewise, just as being born into this world of evil led many to the reign of evil, to become actual (and not merely metaphorical) sinners (v. 19), so does Christ provide the path for becoming actually righteous before the Lord. Jesus turns death into the path for righteousness; the power of sin (death) is transformed into the path of eternal life.
The point here isn't to provide some metaphysical explanation of death, as if one can somehow manufacture the punishment of infants using a magical inherited darkness that makes them damnable. That is pernicious nonsense. The point is that given the reality that all children of Adam are subject to death, in a way AS IF they were being punished for Adam's sin, Christ's power is justly demonstrated in subverting this power of sin entirely to His own power. Christ does what Adam cannot possibly do; He destroys the work of Adam's sin and establishes the path to God through His own divine righteousness. Paul is proclaiming the power of Christ to establish itself, not providing some half-baked theodicy for the damnation of infants. To read Romans 5 as such a theodicy is to debase Paul's theology.