Ok, you should realize, though, that "fair Use" and "derivative works" are two separate legal issues. "Fair Use" says "You can make an exact infringing copy, as long as it's limited in scope and for an allowable purpose", for example, copying a page from a book at the library. "Derivative work" says "Your original creation is similar or derived in some way from this other copyrighted work. Is it close enough to be infringing?" For example, I can write a novel about an English spy, but I can't call him James Bond, 007. How close two works have to be to infringe is very hard to say, and basically is up to a judge. In some cases, even the general "look and feel" of something may be enough for an infringement (eg. HR Pufinstuff vs. McDonaldland). There are also legal exceptions for parody works, but I'm not sure if a morph would qualify or not.
The one thing that you can definitely say about copyright law is that there are very few absolutes.