With respect to Luke's training, here's my take on it (source=
http://www.blueharvest.net/).
The Empire Strikes Back said:
YODA: Stopped they must be. On this depends. Only a fully trained Jedi Knight with the Force as his ally will conquer Vader and his Emperor. If you end your training now, if you choose the quick and easy path, as Vader did, you will become an agent of evil.
If Luke ends his training
and chooses the quick and easy path
in the way that Vader did, he will become an agent of evil. Evidently, Luke did all these bad things
except do them exactly the way Vader them. He did not give in to the onslaught he faced, and was able to back out (or rather fall off a cliff) in time, which was
not an easy thing to do, so he skates. Luke is not a Jedi, and although he will eventually defeat Vader and become one, he will not defeat the Emperor. Vader ends up doing that.
Return of the Jedi said:
LUKE
But I need your help. I've come back to complete the training.
YODA
No more training do you require. Already know you that which you need.
Yoda sighs, and lies back on his bed.
LUKE
Then I am a Jedi?
YODA (shakes his head)
Ohhh. Not yet. One thing remains: Vader. You must confront Vader. Then, only then, a Jedi will you be. And confront him you will.
Luke knows that which he needs to know,
but he is not yet a Jedi. Indeed, it will be
Vader who destroys the Emperor; Luke is not powerful enough to destroy the Emperor himself. He is totally clueless about Force lightning.
What Luke needs to do is save Vader, not directly defeat the Emperor, and he doesn't need to be a Jedi to begin that quest. Completing this quest of saving Vader will require Luke to stay on the good side. It will require that Luke wield enough power, control it, and not be consumed by it, for him to become a Jedi.