1 20-40 in total as a result of another 10-20 when equipping full set
1 Character screen adds damage after % Damage is applied, when it is actually added before % Damage is applied.  Attack damage penalty does not apply to damage added by Blade Sentinel and Blade Fury.
1 Character screen adds damage after % Damage, but it is only applied after damage has been applied to the target. Consequently it does not increase damage but is still taken into account when calculating life returned by % Life Stolen per hit and Life Tap, and mana returned by % Mana Stolen per hit: this means life and mana can be returned even when attacking an Immune to Physical target (provided Life Tap is cast or the target has > 0% drain effectiveness).