Allegedly it does, it requires more testing though, from what I've experienced and heard around hte community, it affects melee, sorcery AND pyromancy. Although patches and additional research and experimentation may contradict that
It may be since your weapon's durability wears down as long as it is inside of an entity such as an enemy or breakable item (barrels, crates, etc.) and if you're staggering your enemies, they won't be able to get away as quick, and your weapon is inside of their hitbox for longer, resulting in durability wearing down quicker. This is just a theory though.
Hmm. That would also mean that backstabs consume more durability then regular attacks. It would be weird if it was programmed like that.
I think it's more than likely negated considering that backstabbing is a special animation, and not a normal attack.