None of these things are plot holes. Why is it unbelievable that Obi Wan would keep the light saber? You might not have, but that doesn’t mean Obi Wan wouldn’t. Who is Luke’s family supposed to be? Yes, Obi Wan still dresses like a Jedi when he lives as a hermit. Vader is subject to the emperor’s wishes. It’s believable that tarken is very effective and therefore the emperor sees how valuable he is and he could outrank the emperor’s useful idiot attack dog.