It's an interesting philosophical question, but intent is something that judges are much better at deciding than computer programs. And laws are generally written to be executed by judges (or, by extension, the threat of litigation).

Fun to think about, though. Especially if you start thinking about writing alternate endings, etc.

It would fall under the same rules as fair use. If the modified file is close enough to the original that it could deprive the creator of a sale, then it is certainly infringement. If you made it so different that it would not deprive the creator of a sale (for example, if you re-arranged every single word so that the words in the book were in an entirely random order), that would most likely be fair use.

It's not quite that simple, something can be a derivative work even if none of the original content remains and it's purpose changes dramatically. Generally speaking fan-fiction infringes copy-write when someone trys to make money off of it. However, the same work may be free and clear if you don't try and make money from it.

Teachers can generally copy something to use in class as an example. However, they may not copy the same content every year after that. Same action, but slightly different context.

If you start with the original work, you'd have to pretty much totally change it for it not to be considered a derivative work.

It gets even more interesting if you use right to "quote" small parts of books.