Jumping straight to Finn, I do remember something of an off-handed comment he made while talking to Ray (?) stating he was taken from his family. I'm guessing, without knowing more than what's presented on-screen, that he was always going to rebel. As for Ray it is something of a plothole but at the same time we just don't know enough at the moment to make a conclusion.
I'm currently sticking with evidence provided throughout the movie: 1. Kylo was torn in several different directions and as a result didn't really have his foot firmly planted on one side or the other. He did things that were considerably Dark Side but he's conflicted, thus emotionally compromised to a degree. 2. He underestimated Rey. He hadn't been challenged by anyone with the force for what we can presume to be years - at least the amount of the time elapsed from the ending of RotJ, more or less.
As for your Vader-related questions: First Order Ex Machina. As far as his lightsaber, it's unstable so much so that it's been presented that his might just be a fair bit more deadly as a result. Now as for why Finn isn't dead it depends on how deep he was cut (I'm over-analyzing a killing stroke from a
lightsaber 
). As things look I'm left to assume Finn will have some kind of cybernetic implant alongside his spine because there's next to no way he didn't suffer severe trauma from that. Should he be dead? Probably.
I can't remember if this is mentioned but the precursor to the First Order was him joining the Knights of Ren. I can't remember exact details on that but I do remember his name surfacing from that venture.