However, I'm not sure Iran's nuclear interests are defensive.
Of course it is. Iran may talk a big game, but do you REALLY think they are going to risk all out obliteration. That would open the door to their complete unilateral annihilation. I'm sure they realize this.
THEM taking a first step would be tantamount to suicide.
If they have such a thing in operation, it IS because they feel a threat exists.