And unfortunately, with Sifo Dyas dead, you will never know why he ordered the clones.
I prefer the Lucas explaination. First he's fighting an old man (who's last saber fight was 20 years ago). Then he's fighting a kid (who just got a lightsaber). Luke doesn't have the experience that Obi-Wan had. Obi-wan still has the skill but he doesn't have the stamina he had 20 years ago. He can't do a long fight like when he faced Anakin-Vader the last time (ep. 3).
yeah that sucks, I would of like to know why he did it.
About the vader fighting, it makes sense, remeber in ESB, when he is fighting luke, he isnt really going on all, until Luke manages to hit him in the arm, and then vader gets pissed and not 2 seconds later, BAAAM, lukes hand is gone.