I suppose you mean Sawtooth Knife. I don't remember my own experience with this skill, but if it does not backstab, it could be explained either by the fact that it does not scale with weapon or it has a specific damage type (piercing).
Whirlwind still backstabs as most other Warfare skills do.
Yes you are correct about the name. It does scale with weapon damage. It is piercing damage, but it most definitely crits. You can force a crit with Enrage if you want to check for yourself. Also rangers piercing attack crits as well, so I am not sure where you are getting the info about type of damage aspect.
