There's a pair of gloves in game that increase your "Backstab" damage by x2. Anyone confirmed whether this has any effect on the talent "Assassins Blade" (as some suggest)? For that matter, are Daggers and One-handers considered the same thing mechanically? Anyone ever level up one-hander when wielding a dagger?

Yes, the two will stack. So the x15 multiplier will be x30 with the gloves. And yes, I level up 1 hand with daggers, though not very often because everything dies in one backstab.