Lucas had the Original SW planned out as one movie, and I feel he had 'ideas' for sequels, but nothing in stone. I believe he wrote the Original SW, and all of the other cool ideas he couldn't fit in a 2 hour movie, he put in his little binder: maybe they get caught in a space slug which people think we'll be an asteroid, maybe someone will be frozen in carbonite, but won't be dead, maybe we can have this huge land battle on this forest planet, etc.

Did he have backstory notes when writing SW in 1975? Probably had that Darth Vader/ObiWan fought and Vader fell into Lava, probably had the Empire taking over the galaxy, and The Emperor ends up ruling. Again, just basic notes to write the first movie.

Did he have anything planned out? NO WAY!!!! I will be honest, it doesn't bother me that he didn't, heck I love ESB, but that movie was probably written in the context AFTER 1977. I just get annoyed that Lucas won't admit that he didn't, as I would love to see a documentary about Lucas stating when they decided to make Vader/Luke's father into one person, maybe it hit him 3 in the morning in 1978, "Hey Marcia, I got it!!!! Darth Vader is Lukes father!!!!!"