Even if that’s so, the reading would have value. But I don’t think it’s opposed to Paul’s thought because he commonly restates a collective truth as singular, and in Romans he already made the case for human creation and sinfulness in plural terms before he recasts it in relation to “one man.”
If Paul took Adam as only being a literal man, it would be similar to how he assumes a three-tiered cosmology (such as “in heaven and on earth and under the earth”). We can keep the message while shading his words to fit our own cosmology.
But I think it’s more likely Paul understood Adam as both the first man and all humanity. That was the traditional view, and until Augustine nobody seemed fussed to show how both could be literally true at the same time. (Augustine’s creative solution – that every human lived in some real way in the first man’s privates – hasn’t aged well.) So I expect our culture has more of an issue with Adam being humanity than Paul.
Also, Paul loved to restate a shared experience or truth in singular terms to stress its unity. All our baptisms are the “one baptism”; all that Christ offers is our “one hope.” All believers are Christ’s bride, or members of his body, or stones that form one temple. Even if Paul isn’t doing something similar in Romans 5, it’s not because this way of speaking was foreign to him.
But I do think there’s good reason to see him doing that in Romans – as long as we don’t treat the second half of chapter 5 as the start of the letter. In Romans 1, Paul discusses creation and fall without mention of Adam, Eve, a serpent or a tree of knowledge. But while Eden’s symbols are missing, the story is the same. God creates the world and is revealed to humanity (1:20). Humans feign wisdom while actually turning from their creator to follow another creature in rebellion (1:22–25). They know this warrants their death (1:32). Eden is retold as the story of humanity, not just one couple.
Romans 2–3 shows the problem isn’t merely people back then or gentiles or Greeks – the problem is simply people. Those given advantages, such as the Jews, didn’t fare better. “There is no one who is righteous,” Paul concludes, “all have sinned and fall short of the glory of God” (3:10, 23). Then, Romans 4 discusses heredity in a discussion of Abraham: we show our true ancestry by who we follow, not who we’re born to (4:11–12, 16).
This context informs Paul’s talk about Adam in chapter 5. The Adam paragraph isn’t about showing we’re all created by God or all sinners or all need a saviour – that’s all been done. Instead, Paul restates earlier themes in the language of ancestry. Just as he discussed active faith in terms of being Abraham’s children, now he restates our universal sinfulness as what makes us Adam’s children. By sinning we join ourselves to Adam, allowing the one man’s trespass to condemn us all, allowing death to exercise dominion throughout Adam – throughout humanity. Ancestry language blurs into identification language; being “from Adam” blurs into being “in Adam,” just as Paul says explicitly in 1 Corinthians 15.
I think it’s because Paul had a layered understanding of Adam that he could use him in different ways, sometimes as a person distinct from Eve and sometimes collapsing the couple into one, sometimes to speak of ancestry through imitation and other times of identity. Had Paul just viewed Adam as the first man, I don’t think he would have been so free with him.