No, it's not simple at all.
Universal's rights extend to any characters currently in use as well as those characters' "families." For example, the Dr. Doom attraction means that Universal obviously holds the rights to Dr. Doom, but Doom is also the arch-nemesis of the Fantastic Four, so Disney can't use the Fantastic Four. Universals rights extend (more or less) over the X-Men, the Fantastic Four, and the Avengers. The confusion is that, throughout Marvel's history, just about every character has been an Avenger at some point.