Actually the NJO story arc was watched over by Lucasfilm. The big writers promoters publishers etc of the series had a huge conference and decided what things would happen in the series. A death of a major player was one decided event, originally it was going to be Luke, but Lucas said Luke can't die in this series. There were a couple other ideas that Lucas nixed as well. Chewbacca was decided on and Lucas agreed that was okay.
As for the authors they were told start here and make sure this and this happens and it this way, and the rest was up to them. It would be impossible for a book to come out every few months without everyone working together on it. As for the EU in general Lucasfilm now takes a big hand in it so everything that happens in the EU goes through them and I think that includes Uncle George in almost if not all cases.
And one last little pet peeve of mine, why is it people who don't like the EU feel the need to constantly say, "If its not in the movie it didn't happen"? Maybe I woke up in a different universe than the one I went to sleep in last night. But in that universe NONE of this stuff happened...