Of course immediate interrupts turn back the clock. They turn back the clock for the players at the table. The DM has to re-adjudicate the situation because the situation has changed. If the hit no longer hits, then it IS turning back the clock. Not from the perspective of the PC, but from the perspective of the players.
From a practical standpoint at the table, I agree. I was referring to character/narrative view of not turning back time
Void to hit resolution. How is this calculated? The equation is used again to calculate to hit. Previous to hit plus previous pluses is compared to previous AC plus the II effect +4 bonus to AC.
Void drop to zero resolution. How is this calculated? The equation is used again to calculate damage. Previous damage done from previous attack is subtracted from previous hit points plus the II effect additional hit points.
You cannot change how you adjudicate the II from one to the other and state that you are correct.
Either you re-calculate how the trigger was determined in every case, or you are not being consistent and you are changing one example from an II to an IR.
Sorry dude, but your method cannot be called an immediate interrupt if you do the healing after the trigger is already set in stone. You have to re-calculate if the equation(s) to get to the trigger based on the effect of the interrupt and see if the trigger is still applicable or not.
What happens when you are hit?
You resolve the effects of being hit (calculate and apply damage, apply riders, etc).
An Immediate Interrupt jumps in when its trigger occurs, taking place before the trigger
finishes. If an interrupt invalidates a triggering action, the triggering action is lost. From the rules compendium page 195.
The first sentence seems pretty clear to me that interrupts happen after the triggering event has happened. It then "jumps in" suspending the resolution of the trigger.
To me the second sentence means, after the interrupt is finished, reevaluate the trigger, if it is no longer valid because of the interrupt effect, the triggering action is lost.
So at the table Shield looks like this:
DM: The Ogre bashes you with his club (rolls) 22 vs AC, I believe that hits. (starts to roll damage to resolve the hit)
Wizard PC: I'm going to use Shield my AC is now 23
DM: Ok, your Shield manifests absorbing the Ogre's blow (last bit is pure narrative fluff).
If you believe that comparing the attack to defense is the completion of resolving "You are hit" then your interpretation is correct and the Interrupt has to happen before the trigger. I believe the verbiage of the interrupt rule quoted above requires that we shoehorn an interrupt in between the trigger occurrence and the effects of that occurrence. Play it how ever you like at your table.
Ask yourself the following question. What happens with the following immediate interrupts?
Trigger: You are hit by an attack
Effect: The attacking foe is stunned until the end of your next turn.
Trigger: You drop to 0 hit points or fewer
Effect: The attacking foe is stunned until the end of your next turn.
Why would the first one completely negate the attack and the second one not completely negate the attack? Give the explicit rules quote to support your answer, don't just make it up.
The trigger "You drop to 0 hit points or fewer" is not set in stone. It's determined by the result of the immediate interrupt, just like "You are hit by an attack" is not set in stone.
Page 215 of the Compendium: (enumerated steps of making an attack)
5. When an attacks hits, it usually deals damage, and many attacks produce some other effect, such as forced movement or a condition. An attack power's description specifies what happens on a hit. Most attack powers do nothing on a miss, but some specify an effect such as half damage, on a miss.
So, the consequences of taking the damage, unless specified by the attack's power description, are not part of resolving the attack. Therefore the trigger "you drop to 0 hit points or fewer" happens after the attack has finished resolving and it can't invalidate the attack. You are dying and your opponent is stunned.
If the trigger were "you take damage" then the attack power, which specifies the damage in its description, would not have finished resolving so the interrupt would stun the opponent, and the stun would invalidate the attack, because a stunned creature can't take an action.
Page 260 of the Compendium (Dying and Death)
Dying: When an adventurer's hit points drop to 0 or fewer, he or she falls unconscious and is dying.
This reads to me as a clear event and resolution sequence. You drop to 0 or fewer hit points, you fall unconscious, you are dying.
With You drop to 0 or fewer hit points as a trigger, the interrupt fits into the sequence between dropping to 0 or fewer hit points and you fall unconscious. If the result of the trigger is you are no longer below zero hit points you don't fall unconscious.
Again, feel free to interpret the above rules differently but stop telling me my interpretation is wrong. It is a perfectly valid interpretation of the quoted rules.