If an early (or possibly orthodox) date for the authorship of the Pauline epistles is posited (ie 50-60 CE), and a late date for Luke-Acts is simultaneously preferred (ie 100-125 CE), how is the eyewitness-like nature of Acts best explained?
First of all I would suggest there is not an "eyewitness-like nature" of Acts (apart from the occasional use of the first person plural, which many authors use to create a more personal atmosphere to the story) and that perception arises more out of a wish that it be so. There are some parallels, but no more so than one would expect from a writer who had access to at least some of Paul's epistles and perhaps knowledge of what the Christian community of his time believed about Paul.
The (three versions of) story in Acts describing Paul's conversion on the road to Damascus reads more like a quotation from the Bacchae by Euripides than the brief description that Paul gave of his own conversion, and Paul's miraculous prison escape, which he never mentions in the epistles, also seems to be from the Bacchae. In Galatians 1:16-17, Paul says that after it pleased God to reveal "his son in me," he went to Arabia (Petrea), but Acts says that after his dramatic conversion, he was taken, blind and helpless, to Damascus. Acts of the Apostles describes three distinct missionary journeys undertaken by Paul, but as Raymond E. Brown points out in An Introduction to the New Testament, page 431, Paul gives no information about the first missionary journey in his undisputed letters. Even the second and third journeys are difficult to harmonise with Paul's own accounts.
If an early (or possibly orthodox) date for the authorship of the Pauline epistles is posited (ie 50-60 CE), and a late date for Luke-Acts is simultaneously preferred (ie 100-125 CE), then the parallels are few enough and general enough to be explained by the author of Acts having access to copies of Paul's epistles, which were becoming widely circulated at this time.