Rayla; you're absolutely right about the Clone Wars! Explain to me who decides who killed Gallia? The only person who decides is Lucas. If he states who killed her then it'd be fact. But until Lucas actually says so HIMSELF, then it's up to you. That's how I'd describe being able to use both sides of the force. Until Lucas SAYS (and not through a character because characters aren't all knowing) that using both sides is impossible, then it is possible. Has Lucas, or any writer ever SAID that it's impossible? Just because some character in a book says it's impossible, doesn't mean it's impossible.
I think Lucas and Chee have said so. That would make it G-Canon.
EDIT: Note that I said I think. I'll have to do research or someone more knowledgeable in this area would have to answer you. Rayla is pretty much the defender of canon so she would be better suited to answering that question.