Ancient History writers often had to invent speeches. Obviously, they didn't have live audio recording. But neither did they have to purely invent. Writers could reconstruct speeches from interviewing the hearers. And sometimes more.
Our first point, then, is that ancient textual speeches aren't always pure-fact or pure-fiction. Like most human claims, they bear shades of the truth. Thucydides' work was generally much less fictitious than Herododus' was. Likewise, Polybius also built from first hand experience and live witness testimony. And in the 2nd century AD, Dio Cassius had more than enough evidence on Augustus' actual thoughts and opinions to have crafted some pretty believable content - at least - into the speeches he wrote for Augustus.
These were hardly uniform procedures. In early histories, Josephus has people say things he couldn't possibly have known about. But when Josephus worked from Nicolas of Damascus' first hand accounts, his renderings of Nicolas' own speeches must have been fairly close (in many respects) to the actual speeches that Nicolas actually gave.
Not that Nick-o couldn't have embellished his own record to make himself look good, of course: "And then I told Caesar..." Propaganda, no doubt, always begs to be doubted. But how much can ever be proven? Or disproven?
I will admit this much right from the start: Stephen's speech in Chapter 7 of Acts absolutely does fulfill literary and theological needs fitting with Luke's agenda. There's no question about that.
But I will argue this much right to the end: Having agendas doesn't force one to fabricate details. If you happen to have all the cards, then you simply have no need to bluff.
Now. Even supposing that, kindly, as a plausible option, many questions remain: precisely HOW could Stephen's speech be historical? How much of what Luke wrote could possibly be close, at all, to whatever Stephen actually said? How could Paul, as Luke's source, have remembered the content of one particular speech for so many long, drama-packed years? For that matter, why would that speech have been memorable at all, at the time, for the persecutorial Paul?
In other words - if the speech truly happened, and if it merely happened to give Luke such a perfect display piece for what his composition required at that point - then, even still - how on Earth did Paul remember [at least the core and basic thrust of] what Stephen said, when Stephen was so much Paul's enemy at that point? How could Paul have remembered so many particular elements [of what could easily have seemed to be, apart from Luke's surrounding literary context, a very generalized survey of Jewish history]? And - to the ultimate point of this inquiry - why on Earth would Stephen have spoken that [or something or anything like that] particular speech, in the first place?
I'm completely assuming, of course, that the event sequence of Acts 6-7 is historical. No, I won't bother defending that. If you can't entertain this one assumption for the sake of the rest of my argument, that's totally fine. Really, have a nice day. But for anyone game enough to consider these questions, stay tuned.
Or maybe re-read my last post for a hint at what I'll be suggesting...