The recent cracked photoplasty on Plot Holes You Never Noticed in Famous Movie Scenes
features several Star Wars related items. I hereby attempt to rationalize them.
Easy. Deep inside he wanted Luke to get away. Hence the whole redemption thing at the end of the third movie.
Also easy. He's not
"required to passively watch while the Emperor murders all his friends and crushes the Rebellion". The emperor just totally psyched him out into believing it. You know, weak minds and all that.
Um. Hold on, I'll think of something ...
OK, got it. Obi Wan is the stronger Jedi, mentally, and used this power to (mostly) block his presence from Vader.
Yeah I got nothin'.