The letter [Romans] has spawned an immense corpus of scholarship, largely because Paul is not entirely clear... he seems to be working it out as he goes along.
When Paul uses scripture in his letters, only in part can it be seen as an abstract theological reflection. Primarily its use must be seen as rhetorical; he cites scripture because he thinks that it will prove his point to his audience.
... some of his interpretations are so strained that he seems to be counting on the fact that his audience gives authority to scripture but does not actually know it well enough to take apart his arguments.
Few listeners would have heard a verse [cited by Paul], thought that it conflicted with a version of the verse that they knew, and then gone to consult the appropriate written text to check. "Scripture" primarily circulated in oral form.
We do not know if Paul's audiences would have understood or been swayed by what would have seemed to them his odd use of scripture. It may well have been Paul's charisma that gave him authority in these communities rather than his intellectual virtuosity.
Michael Satlow's book on How the Bible Became Holy deserves a detailed review, and I want to focus in a later posting on his intriguing account of the separate development of normative scripture in both Hasmonean Judea and the Diaspora. I've been chugging my way through, a chapter at a time, and become riveted by his reconstruction. As an appetiser, and somewhat peripheral to his main thrust, these "don't hold back" statements all come from chapter 12, "Paul: Jerusalem and Abroad, 37-66 CE".