NOT to be argumentative.... but a) is kind of a sloppy answer, if it's the truth.
"Why are their plot holes in your story, sir?"
"Uh, magic."
One can write off ANYTHING by saying it's the 'will of the Force', that's a cheap way out.
As for b), I have to disagree with the assessment of Luke's 'decision of his life'. Luke didn't stop because Vader was suddenly an unarmed person (BTW.... FABULOUS pun, intentional or not), Luke stopped because he was enraged at the thought of Vader using Leia, and Luke's rage took him right up to the edge of the Dark Side. It helped him beat Vader. Luke is FURIOUS when he fights Vader back to the reactor shaft.
And the Moment of Truth is when he looks at Vader's stump and then his own mechanical hand. It's not about the Jedi Code or killing an unarmed opponent (Mace Windu, paragon of Jedi virtue, was fully prepeared to kill Palpatine, and Palpy was unarmed).... Luke uses anger, fear, aggression, the Dark Side to defeat Vader and JUST when he's ready to kill Vader, in anger, he looks at how he's becoming JUST like Vader. The mechanical limbs are a metaphor.
I'm not trying to be uncool, or argumentative.... just trying to discuss. I totally respect your right to your own opinion, sir.
he he I actually agree with what you wrote too. I don't see how the two are incompatible, frankly. But whatever the reasons, it was THE decision Luke had to make to become a Jedi and had he not started off on the right track, being raised completely clueless of his parentage, he may not have made the right decision.