“Wherefore, as by one man sin entered into the world, and death by sin…” Rom5:12a KJV
Paul’s comments in Romans 5 build from his foundation established in the preceding chapters. All men have the testimony of Creation but do not necessarily respond (Romans1). Jews broke the law they boasted in (Romans2). Consequently, all men, whether they specifically have access to the oracles of God or not, justify their mortality as “all have sinned and come short of the glory of God” Romans 3:23. The way of salvation is righteous through faith, as demonstrated by Abraham (Romans 4).
Paul’s point is contrasting Adam with Christ. Adam and his transgression are “type of the coming one” Romans 5:14. NET. This is significant. We are looking at a contrast between two ways. Even though men didn’t commit the precise sin of Adam, those who lived prior to the Law of Moses are still considered sinners (“made sinners” v19 which is better understood as ‘accounted’), in the same way that through Christ’s righteousness many can be accounted as righteous. Paul explicitly says they had no law but they experience death as sinners ie they die and will end up/stay dead (v eternal life which comes through being in Christ). The passage in v12 must be understood in the context of the contrast which forms Paul’s point.
It is not necessary for Adam to have been amortal for Paul’s argument to be perfectly valid. The wages of sin for Adam were and remain death (Romans 6:23). His life in the garden was contingent on his ongoing obedience to the law of God which, once broken, invoked the certainty of his fate and sealed his destiny to paraphrase Bro Roberts 1869 comments.
Paul’s point through the passage to v21 is to demonstrate that in Adam (the wording is not ‘descended from’) all are accounted as sinners who die, but in Christ (again literal genealogical descent is not in view) grace leads to eternal life. Paul is explaining too that God’s righteousness isn’t compromised by the death of a man who never was accountable to God (never had the law) – why? Because all men are naturally in Adam and Adam proved when there is a law we will sin/break it. Like Christ, Adam was a federal head, a representative individual who demonstrated the righteousness of God (in Adam’s case in the death of all men).
 Roberts, ‘Apparent Contradictions Reconciled’, The Christadelphian (6.62.243), 1869