OK, there are official levels of canon, set forth by Lucasfilm in 1991, around the time Heir to the Empire was written. In this system, the movies, the six episodes, that is, take precedence over all other works. Lucasfilm considers canon anything that does not conflict the movies. All works are equal, if they don't contradict each other. This makes the entire saga one big, complete story. In this case, although it is implied that Mace died, it was never stated in the movie, or shown. Yes, that does leave it open. I'm guessing Lucas didn't think about such small details when he wrote the story. The book says he died. In this case, the movie can't take precedence, because it doesn't hold a position, it only implies what happened. It does not contradict the level of canon which is right under it, which are the novelizations of the films. Since there is no contradiction, we must find our answer in the book. What I meant by Fett surviving was similar. ROTJ never specifically said he died. It just showed him falling into the sarlac. Since the movie doesn't specifically tell what happened, we must look down a level at the EU, which says he lived, therefor, he lived. The bottom line: All things that don't contradict the movies and higher levels of canon are part of the actual star wars story, and "happened," film or not. Now, fan fiction is not part of the story of Lucasfilm at all, so it cannot explain anything in star wars, no matter if the EU explains something or not. Fan Fiction is not unpublished EU material by the way, because it is not sanctioned by Lucasfilm. It is mere fan speculation and ideas, not the ideas of those officially given the opportunity to tell part of the official star wars story. It all has to be looked at like this: with a historical event, you can only rely on actual records and people invilved in the event to solve what actually happened. Written records take precedence over the reports from the people, let's say. These are official reports. But fan fiction is the same as if someone decided they knew what happened in the past, but they just made it up, see what I'm saying? According to Lucasfilm, here are the Star Wars levels of officiality:

1. The current version of the episodes
2. Earlier versions of the episodes
3. The novelizations of the episodes
4. The radio dramas of the episodes
5. The EU books and other media that don't contradict the films
6. The EU books and other media that do contradict the films

How do I know this? I listened to an intense two-hour canon lecture on Chrono Radio.