From Trademark and Copyright Law:
If you are a television news producer or documentary filmmaker, you have almost certainly faced this issue: You are putting together a story about a past event, and you want to make the point that this past event was once the subject of media coverage. The easiest way to do that is to show some of that media coverage, for example, by including a short clip from the evening news or by panning across a newspaper article headline. Are you allowed to do that? Are you infringing the copyright in that news clip or in that article?
When I was a video editor (and not yet a lawyer), I was regularly told by other non-lawyers that such use was permissible as long as it didn’t violate the “seven-second rule.” This apocryphal safe harbor provided that, if you show a copyrighted work for fewer than seven seconds, then you were protected either by the fair use doctrine or by the equally mythical doctrine of “nobody cares.” Why do intelligent professionals pass on such fables to each other? Perhaps it is because, when you are knee-deep in raw footage and working on a deadline, you crave certainty.
In fact, such certainty rarely exists in copyright law. There is no “seven-second rule;” there is no “nobody cares” doctrine; and advanced reliance on predictions about fair use (a notoriously uncertain inquiry) is risky at best.
. . . .
The closest thing in actual copyright law to the “seven-second rule” fable is the de minimis defense, which derives from the legal maxim “de minimis non curat lex” (the law does not concern itself with trifles). In copyright law, the de minimis doctrine is not truly a separate defense but an extension of the elements of copyright infringement: to prove copyright infringement, you must prove copying, which can be done with a showing that the copyrighted work and the allegedly infringing work are “substantially similar.” Where copying has occurred but it is so trivial that the works cannot really be said to be substantially similar, the copying is considered to be de minimis and therefore non-actionable.
Link to the rest at Trademark and Copyright Law
The OP provides some good examples of why questions like, “Can I include this in my novel?” are not always easy to answer.
If PG were king for a day, he would probably add some safe harbor provisions to copyright law that set clearer boundaries between what uses of others’ original work are and are not permissible.