Steven, you never seem to have answered Bob Prokop's comment, which he has made over and over, that the novel genre didn't exist in the first century.
Second, your use of the word plagiarism is highly anachronistic. At the present time, we are concerned about intellectual property rights, and so we have copyright laws and plagiarism regulations. But in that time, there was no such thing. If I have to grade someone's paper, then I need to know what in the paper is the writer's own idea, and what came from sources. But Luke, for example, isn't writing his gospel for a grade, and he's not trying to take royalties away from Mark. And although some people have made claims on his behalf as a historian, he's not trying to compete with Thucydides. He's trying to get the story out that he believes to be true.
In our century, we are accustomed to reading books in which the writer learns a lot of factual details so as to provide a realistic setting for the work, but the work itself is a work of fiction. So, if you were going to write a novel about someone who was going around, say, the Far East committing crimes, getting arrested, and escaping, then you might, to make the story realistic, go to the various places in the Far East, and visit the police headquarters and court buildings to find out what their police and court procedures might have been, to put in your story. You would find out how they do things in Bangkok, in Rangoon, in Hanoi, and in Ho Chi Minh City, in New Delhi, in Mumbai, in Singapore, and in Lhasa. Of course, if you were doing this today, you'd have the benefits of planes, trains, and automobiles, none of which existed for poor old Luke.
So, if the Gospel of Luke and Acts were both novels, how did he research his novel? In the words of Andrew Lloyd Webber, Israel in 4 BC had no mass communication.