George worked much the same way. He didn’t have the sister thing worked out yet when Yoda spoke of another in Empire. Did Irvin Kershner have to know if Vader really was Luke’s father or not to make that scene work?
I think someone at Lucasfilm will be guiding things. People are not just going to make up whatever they want.
With Kathleen Kennedy at the helm of Lucasfilm, you know it won’t just be all willy nilly. And remember that while VIII and IX will not be written by JJ, he is still the EP on the movies. There will be a steady hand at the tiller. Just because someone won’t recieve a writing credit in the titles doesn’t mean they won’t be present or consulted in order to maintain continuity.