Except that there are no such rules as "the attack statement is separate from the triggering statement". There are just a bunch of rules on how to resolve the entire action. Nothing in those rules indicate that an interrupt can invalidate some of those rules, but not others.
The rule is that an immediate interrupt can invalidate an action.
One rule is that Courageous Insight can not only prevent an attack from hitting the target when the target drops to zero (the triggering statement according to you), but it can also change the target of the hit (and hence in your terminology, the "targeting statement").
There really are no limits to what portion of an action can be negated as long as the conditions of the situation change drastically enough.
Teleporting away can change the range of the attack, even though range is calculated earlier than either the Attack line or the Hit line.
So, why can't healing change the attack bonus of the attack when the Attack line is calculated earlier than the Hit line?
Immediate interrupts interfere with the entire action, not just portions of it. There are no rules like you imply of an attack statement being calculated first and then can never be looked at again for an interrupt. There are many examples of IIs that do just that.