Although it's not much to go on, this interview with James Kahn (the writer of the ROTJ novelization) does at least confirm that Obi-Wan's extended talk to Luke on Dagobah was based of a conversation with Lucas and not of his own making. So it seems that we can take the Lucasfilm sections of the AGTTSWU serious as a pre-PT George Lucas canon.

http://www.starwarsnewsnet.com/2015/10/swnn-interview-james-kahn-author-of-return-of-the-jedi-novelization.html

(This of course still implies a lot of retconning in between the films, but at least we can identify what Lucas intended back in 1983.)