I think that when he started writing his concept for SW, it was too big, so he took part of it and made it into ANH. Then when that was successful, he started plugging more of what he originally came up with into the next two episodes of the classic trilogy, and was even at one point thinking of making the full story a 9 film cycle. But when he got to ESB/ROTJ, he suddenly realized he didn't want to spend that much of the rest of his life making SW movies, and by then he wasn't even sure he wanted to do more than 3 with all the headaches he was going through trying to make them happen, so he took the final resolution of the Emperor being vanquished and pushed it up to the 6th film.
When that happened, all the story treatments he had come up with for an eventual 3rd trilogy were rendered obsolete, because they all ended with the same resolution we got at the end of ROTJ: That the Sith had finally been vanquished. So while he had been thinking about doing a third trilogy, he had never come up with anything beyond the destruction of the Sith. To him, that's where the story ended, and he never took it further.
I also don't see the motivation for Lucas to lie. The man is the master of his own destiny and anwers to no-one. What purpose would lying about this stuff serve?
I also don't see the contradiction between Lucas saying the dark side is more powerful in the short term, because Yoda says it's quicker, easier, more seductive. That makes it more powerful in the short term, but lacking in the long term, which is pretty much what Yoda is saying too as far as I can tell.