1) Reading through the forums, I have the feeling Primordial Vengeance is a huge spell for animists but some wrote it was broken... is it still the case?

2) Is it possible to practice Primordial Vengeance on NPC or, like assassinate, it's only usable against PC?

3) Is there another type of ranger, beside Animist, that can use scrolls?

Scrolls is a default skill for rangers. Some terrains/expertise lose scrolls those. Look at the wiki to see a better run down of who loses/gains what.