Regarding immortal NPC's: I'm not sure if we're going to make certain NPC's "immortal:. I think what we plan to do is just tag the NPC so the player cannot attack it (pick it's pocket, or otherwise piss it off).

Story based RPG's have obvious fail situations. The most obvious is when the player or the party is killed off. They cannot continue the quest, so the game ends.

Killing critical NPC's is similer. Killing Aribeth in NWN is a fail situation, and the story ends.

What would happen if Frodo killed Gandalf right away? It wouldn't be the same story at all.

So kill a critical NPC in Dragon age, and you're option to continue is reduced to nothing.