I don't believe Luke went dark at all. He is neither Kylo, nor Snoke nor has he turned to the dark side.
I think Luke is a pure Jedi Master that did exactly what his former Master (Yoda) did. He went in to hiding.
Jedi even after the death of the Emperor are still very much so hated. I also believe that Luke is NOT the only Jedi out there. Vader didn't get every single Jedi, nor did the "First Order".
I believe how they are going to introduce this movie though is that Luke is the lone Jedi out in there and there is no other. I know for a fact this isn't true though.
As for the helmet. I am pretty sure Luke didn't take it. I am sure that he just left the burnt remains there. Obviously you know the body isn't there. It's gone, just like Yoda and Obi. The suit was probably all melted along with the mask, but also remember that the First Order DID exist in a form during Return of the Jedi. They were used by Vader, and maybe Palpatine to hunt down the Jedi along with various other tasks.
About 1 more month. DAMN I can't wait to see this story and see where they went. Azz, I did see a youtube thing about it might be Chewie who died because there was fur.
Honestly, I'd like for them to kill off Han, Leia and Chewie. Only have Luke left to do what must be done!
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EDIT:
Lucas's version has Luke being all powerful. Lucas said himself that Luke is more powerful than any other force user in the universe. Now, I know JJ and company didn't take ALL of what Lucas said and made a movie, but I am sure they kept certain thoughts.
EDIT2:
Remember, Luke was alone a lot during the original movies. He was in training for....how long before seeing Han, Leia and the others. Years....
He only showed up to save Solo. Force powers he showed right off the bat, raising an GIGANTIC door. That's massive force power. He then force choked 2 pigs....just by waiving his hand just like the Emperor. The dude is nothing to scoff at.