#1) In my experience shadow veil does not stack, nor would I expect it to. With 20% armor and 10% dmg buffs, 4 rogues could have an 80% armor bonus and 40% dmg bonus. That would be OP.

#2) I prefer Shadow piercer over noxious bolt. With the heal buff and the added targets buff, shadow piercer works as a great healing skill, can be used as a sort of AOE, and it can dash you out of tight spots when things get too close for comfort. The heal buff gives a 75% chance to regain 10% of your health for each hit, so with 3 hits per charge, you can spam the skill a few times and regain almost all of your hp.
If you want to gear for PvP, then use noxious bolt. Shadow piercer wont help you much there except to catch up to opponents running away from you. Shadow piercer can also utilize your crit buffs (3 hits = 3 chances to crit). Noxious bolt DOT does not crit.