Okay: first of, all your questions have been answered already at the official site and the forums here
1) He isn't Dark Sided. He is kept under control by Vader by an arm bracelet of some sort. Also, he isn't trained to be DS, just to use the Force to aid him wherever neccesary. That's a contrast with the Jedi, who try to use the Force only when neccesary.
2) He is killing stormtroopers because he can't leave any witnesses. The rule of two is established, which means a Sith Lord can only have one apprentice at a time. Vader has his own apprentice, which he must hide from the Emperor. Also, the Emperor doesn't trust Vader (and vice versa), another motivation to hide him from the Emprire and Emperor.
3) Official site mentioned nothing about your rumor...but the site DOES mention the tale is about redemption, so it could be a lead nevertheless
Hope this helped.