In response to the question, "Is Jackson's work nothing more than an elaborate fan film?": I think it is a part of human nature (or at least in the nature of storytellers) to elaborate upon the retelling of a story, something that has been going on since ancient times. It's what Homer (if there was such a person) was doing with the Iliad and the Odyssey--building upon an existing narrative which existed in the Greek oral tradition.
If anyone's interested, here's an interesting documentary I found on youtube that links ancient storytelling to modern cinema storytelling:
It's part four of a series done by the BBC called "How Art Made the World." You can also buy a legal copy on Amazon for $15 that has the entire series on it and probably isn't nearly as pixelated ;P
It seems easier to dismiss the Hunt For Gollum as a fan film because it's unofficial (stated as such in disclaimer by its own creators) and the small audience size.
The more difficult example to me would be properties with many different "official" versions (disclaimer not needed) all aimed for a very mainstream audience.
I bring this up because I've just finished watching Sherlock, the popular BBC television series re-telling of Sherlock Holmes in modern Britain and comparing it to the semi-recent films, Sherlock Holmes (2009) and Sherlock Holmes: A Game of Shadows (2011)
Both are re-inventions of Sherlock Holmes that are without a doubt not 1:1 mirrors of the novels themselves. Yet, both of these modern re-writes of Sherlock Holmes in new formats has a viewership large enough it may compete with (or dwarf) the current amount of readers of the actual novels themselves, making it quite difficult to dismiss them as "fan films outside of canon"
The interpretation of Sherlock as cold, high-functioning Sociopath by Benedict Cumberbatch when compared to the frolicking fist-brawler Robert Downey Jr. are different to the degree that they must be separate characters. That's even more obvious for the settings as the BBC television version is set in current times with cell phones and hackers and everything rather incompatible with the original settings from the novels and forcefully creating a separate "television series canon"
I wonder if the canon-incompatible setting of the Sherlock television series was deliberately chosen specifically to create a work-space of fresh, new canon possibility which was completely unlinked to the mainstream movies so that they would not be competing against each other in story or audience.
That's true. Some properties seem to split and have multiple equally legitimate canons...maybe at that point you just have to consider them different entities entirely. The property I'm looking at for my project this semester is Howl's Moving Castle. Diana Wynne Jone's books and the Miyazaki animated version are very different, and even have separate yet overlapping fan bases. Of course there are going to be the fans who insist that the movie got everything wrong, and some who like the movie version better. But then some seem to accept both versions for their different merits and somehow accept both versions as cannon--kind of like the Orwellian concept of doublethink, I suppose ;P