Perhaps you could set up something so that when an NPC is invulnerable — for example, where it’s busy doing some unexpected cinematics or still moving to the point where he needs to get to before starting combat — the name is shown in a different color than plain old red? I can’t begin to count the number of times I or other folks have started throwing out abilities (sometimes the big ones) on hostile NPCs only to find out they are ‘invulnerable’ due to storyline reasons.
A good example of this would be in the Norn starting area, where the Svanir are attacking the bear shrine. You get slews of Svanirs marching right up into the fray, but despite looking exactly like all the other combat-ready Svanir, they are all invulnerable up to the point where they reach some magic spot and then begin attacking.
I understand the need to have them invulnerable while getting them from point A to point B, but if it’s a non-game mechanic reason they are invulnerable, having their name be a different color than red — maybe purple or orange or something — would help when trying to prioritize targets and such during heavy melees.