In v14 Paul here raises a logical question. Everyone died from Adam to Moses despite there being no law. They had no instruction about restricted fruit or 10 commandments but died anyway. He doesn’t answer this issue – just observes it for answering in v15.
His point is not actually about Adam – it’s about Christ and the contrast to Adam. So Paul continues the contrast, building with the statement that Adam was a figure a type of the Christ – he is establishing Adam as a typical/representative federal head in contrast to Christ. However the section from v 13-18 is effectively in parenthesis and complex because, as Cranfield notes, Paul has
“hedged [the comparison] with qualifications emphasizing that this is no comparison of like with like but a comparison of two persons who are utterly dissimilar except in respect of the actual point of comparison”
In V15 Paul states that while Adam is a type, there are significant differences in how things operate being in Adam or in Christ. One man’s sin has many – who never had the same access to law/opportunity for failure – accounted as sinners and dead without hope. Far greater than this – by grace through one man (Christ) there is the gift of grace. What is the gift of grace? Eternal life.
In both instances, those in Adam and in Christ there is a timing gap between their ultimate end. Adam & Eve did not immediately die (but were immediately excluded from the tree of life). Similarly being ‘in Christ’ is NOT an immediate lifting of mortality. Those who receive the gift still feel mortality. However death is not their end (even if they do die). So John can say to his mortal readers “these things have I written unto you that believe on the name of the Son of God, that ye may know that ye have eternal life” 1John 5:13. Consequently v15 stands against reading ‘mortality came by sin’ into v12, instead it underlines the comparison of two types/representatives.
V16 & 17 Paul adds another contrast. The judgment on Adam was one man’s sin leading to many being condemned. The word condemnation here means “A decision against someone, a condemnatory judgment”. This phrase contradicts usual exposition asserting mortality was the punishment and meaning of v12. Mortality is not the judgment against men. Saved, unfaithful, wicked and ignorant all die due to mortality, but the wicked, unfaithful and ignorant will stay dead (albeit some rise temporarily to receive their final judgment and then return to death). Condemnation is judgment – a final decision. This again raises the question – how can those who never sinned be subject to condemnation. Paul will answer this question shortly – it’s not his focus (Christ being greater is his focus). So the contrast Paul is making should be clear. One sin led to the condemnation of death. But the gift is received despite many offences/sins on the part of the saved.
V19 Paul says many are “made sinners” is critical here. It should be understood as ‘accounted’ or ‘considered’. We are not sinners because of what Adam did. In the same way, we are not strictly righteous in ourselves because of what Christ did. We are treated as righteous or accounted righteous. This righteousness by association (if I may put it that way) works in reverse with Adam and death coming by sin. Just like in Christ we are considered righteous, so in Adam many are consider/counted as sinners – even those not personally accountable from contact with God’s word. They receive the same outcome/punishment in a federal headship sense. As Paul has already established – all men are incapable of showing God’s glory (Rom 3:23). Adam demonstrated once and for all that all men everywhere will transgress God’s law if given opportunity. Hence the death of any doesn’t contradict God’s righteousness. This is a critical point, interpretations of Paul’s words must work both for both those in Adam and those in Christ.
The passage does not demand direct descent, it is neutral on this point – open to reading either way. The emphasis of the text is that we are “constituted sinners” to echo the Unity Book (which itself is quoting Bro Thomas “The Revealed Mystery” which I don’t have access to). 
V20 So says Paul, many without the law were considered/accounted as sinners in terms of their relationship to death. Once the law becomes available, sin is much more obvious/defined but grace still triumphs. Again this provides further support for the way Paul is using sin through this context.
V21 Sin reigns unto death – grace reigns through righteousness unto eternal life says Paul. The contrast between the two adds to the point of death through sin in this context meaning eternal death/death as the final outcome of the unrighteous/those not in Christ. The verse stands against those who read death as mortality, sin reigning unto mortality is not right – saints and the ignorant are all subject and reigned over by mortality (at least temporarily!). Paul is talking about the two ultimate ends of man. Eternal death or eternal life. Sinners (actual and accounted as such) get eternal death. Those accounted righteous (through the grace of God) get eternal life.
So in summary what is Rom5v12-21 saying and what does it not say?
The passage does not say mortality started with Adam. It says sin (as in deliberate transgression of the law) commenced with Adam and death as a punishment for sin started with Adam. The passage sees Adam as a federal head similar to Christ. In one we are accounted sinners and worthy of death (as punishment for sin despite individual circumstances), in the other we are accounted worthy of eternal life and mortality’s impact on us is temporary.
The passage doesn’t say death by sin comes through descent in Adam. This may be implied but is not explicit. As with life in Christ, the issue is identification not strictly descent.
Romans 5v12-21 does not support or imply Adam was not the first human. My point is that this passage doesn’t conclusively rule it out either.
 Cranfield, C. E. B. (2004). A critical and exegetical commentary on the Epistle to the Romans (p. 273). London; New York: T&T Clark International.
 Carter, J. Unity Book. Page (1963)