Actually they would stack, since they add to different areas. Blade of the Society adds specifically to sneak attack damage, while Dirty fighter adds specifically to flanked. Also the other one being to non-lethal while the others are lethal (unless you specify otherwise)
Because you don't... the action is "step up", it is just extended, as per the first words used "When using the Step Up feat to follow an adjacent foe" so to provoke from following step, you'd have to provoke from step up, which you don't... So it's "yes" to both" or "no" to both, and since step up says no provoking, following step doesn't either.