Copyright generally doesn't cover broad plot structure or tropes, especially components that are considered characteristic of a genre (scènes à faire). Where it gets tricky is deciding when a particular arrangement of plot points and tropes is close enough to be infringing. The lines between rip-off, homage, parody, subversion, and (sub)genre (re)invention aren't especially bright or sharp. As with many other things, how much you can get away with seems to heavily depend on industry influence/standing. I don't think too many people other than Mel Brooks could have made Spaceballs happen, for example.
Nah, that's far too limited and specific. Patents have been extended in scope somehow to cover things like "business models" and algorithms, far more abstract things. Copyright keeps being held to specific sequences of tokens and much more concrete things.
The problem there is that it's incredibily difficult to prove copyright violation / plagiarism; a recent example, there's a Tetris film on Apple TV and the author of a book is suing because they believe it lifted their story after he sent a pre-production copy to the company currently owning the Tetris brand: https://www.theguardian.com/games/2023/aug/09/the-tetris-eff...
Right, but we don't want to just "take it down"; we want to extract rent from the efforts of others by legal means.
To move into the music industry, we might patent the idea of "notes" and thereby gain royalties on all extant music... certainly any future music, what with "prior art" and all...
But arranging tones in families by resonances and using that system to make a written representation of tones for the purposes of later independent reproduction of the sequence of sound? That's a patent-able idea, at least as much as "Method and arrangement for data compression according to the LZ77 algorithm"