1. yes, any model armed with a "power weapon" is armed with a power weapon of your choice, just make sure to model it correctly, so yes you can give your death cult assasins your choice of any two power weapons, and your crusaders your choice of any one power weapon
2. power weapons are not specialist weapons, hence you get +1 attack for haveing any additional close combat weapon
if anyone gives you greif over giveing them power weapons of your choice, notice that the sisters
FAQ specifically changes the wording from "power sword" to "power weapon".
also in my
GK codex which also has these units, the reference is also to "power weapons"
side note, power maul + axe on the death cults for the win!