Nice one, and eeeewwwww...
I can't answer to Lucas's direction of Alec Guinness, I'm VERY sorry to say. (Wish I'd been there. Man...) But, in the course of my thoughts about the series, I've been really amazed at how easy it has been for me to explain several things. I'd like to believe that Lucas knew what he was doing. Because the alternative -- that he's quite simply the luckiest blundering geek on the planet -- is amazing and somewhat horrifying, and might go a long way toward proving the existence of divine influence. In short, the Force was with him.
I'm not one of those people who thinks that Lucas is without flaw. He can't write dialogue. He's awkward with character interaction. And his use of comic relief is questionable at best.
But there's no denying that, in the things he does well, he's at the top of the game. And Star Wars, in all its epic immensity, is something that Lucas does well. Given his meticulous attention to detail, I can't believe that he wouldn't catch even the most remote inconsistency and find some way to counter it. He may not counter it in a way that you or I or any other fan would prefer. But I'm okay with that. It's his saga.
What I'm suggesting is, Lucas likely wrote himself into a corner a time or two, sure. But I've no doubt that he saw those corners for what they were, and deftly found ways to write himself out of them.