Yeah, I recently thought this could have worked better using an element from the older version of this backstory, or at least a version that Mark Hamill recounted back in the day.

He said that George told him that Obi-Wan, Father Skywalker, and Darth Vader all get together for a duel (you know, why not? ^_~), Father Skywalker is killed, and Obi-Wan eventually gets Darth Vader fried, but Obi-Wan is forced to retreat because the Emperor shows up.

Obviously, since the whole duel concept was thought up back when Obi-Wan was avenging Father Skywalker's death at the hand of Darth Vader, Obi-Wan leaving him to die would make a lot more sense. But this interesting variation with all three characters being present, and then the Emperor showing up... well, that would have worked for this. Vader gets set on fire, and Obi-Wan's going to put him out of his misery, but then Palpatine shows up and tries to lightning bolt him, so Obi-Wan is forced to flee so Palpatine can do his experiments.

Unfortunately, George wanted to have the Vader/Obi-Wan duel intercut with the Palpatine/Yoda duel, so it's still pretty amazing that Palpatine gets to Mustafar as fast as he does.