Heh, was alittle less confusing a few years ago when the official policy of "what is canon" was:
- Movie Novels
- Radio Dramas
In that order, movies taking precendence over everything of course.
The rest, if official, was EU.
About Coruscant, I believe George had a whole list of names for the planet, but since he never got to depict it in the 3 first movies, he never decided on one. Coruscant was part of that list, and Zahn picked it out.