The problem is that the trigger is not a set point in time for this power.
Set point in time? What does this mean? There are no set points in time when resolving an attack action. Where do you find rules for this?
There are a series of steps that determine what happens when an attack occurs. Sometimes we skip over some of these or do them in a slightly different order, but it's not just a few steps that are "set points in time". They might be set points in time with respect to how a given set of players calculate what happens, but there are no rules for when each step occurs and they don't get locked in stone once a step is over. For example, steps 3 and 4 below can often be switched from gaming group to gaming group.
1) Choosing Targets. If you want to use a power against an enemy, the enemy must be within the range of your power, and you have to be able to target the enemy.
2) Determine effects that occur before attack.
3) Determine Defense. Defense +- modifiers.
4) Determine Attack Roll. 1d20 +- modifiers.
5) Determine if the attack criticals.
6) Determine if the attack hits.
7) Determine damage. Damage +- modifiers.
8) Determine new hit points.
9) Determine if the PC changes state like going below zero or being pushed to a different square.
10) Make a hazard roll if target moves into hazardous terrain or over a cliff.
11) Determine effects that occur after attack resolution.
12) If more than one attack, re-do those steps that are necessary in the order that they are necessary. For example, two attacks against the same PC would resolve before the first attack pushes the PC away.
The action is all of these steps and can be partially or totally negated by an immediate interrupt interfering with any of them. Effectively, all of the steps have to be re-done in order to determine what happened. It's not going back in time, it's changing the metagame calculations based on what powers are in play, including the immediate interrupt.
Let's take an example:
Yensurros, Ghostly Malison attacks a PC with Double Attack and hits and pushes a PC. A different PC uses Evade Attack to teleport the first PC away if he gets hit.
Step #1 above is negated because Yensurros is no longer in range to do the attack at all.
The second PC cannot let the first attack hit and do damage, and then teleport the PC away when the second attack hits. The attack resolution steps are not set points in time. There are no discrete points in time within the action unless the action states otherwise. Just like the first attack cannot push the PC away before the second attack occurs. Both attacks are resolved, even though the power does something in addition to just doing damage.
Let's take a different example:
A Silt Runner Inciter shift and attacks a PC with Incite Fury and hits. A different PC uses Evade Attack to teleport the first PC away if he gets hit.
The shift portion of the Incite Fury power (step #2) still occurs. But, step #1 above still fails. It's a partial interruption of the power.
The Shield spell would interrupt step #6 and recalculate step #3.
Bear's Endurance would interrupt step #9 and recalculate step #8.
People can view this as "going back in time", but it really isn't. It's starting from scratch with all of the powers that are currently in play for this action and determining what happens.
It's all one action. Any immediate interrupt can interfere with any of the steps and force a reevaluation of any of the other steps, resulting in any or all of the other steps to fail or not even be used at all.
If an immediate interrupt power changes the result on an action, nothing stops a different PC or NPC from throwing a second (or third or fourth) immediate interrupt power to change the result again. For example, if a monster had an immediate interrupt that allowed an ally to teleport next to an enemy if that enemy teleports (if such a power exists), then Evade Attack would move the PC away and the monster would teleport his ally next to the teleporting PC and the original hit would resolve normally.
Each time, the players have to stop and reevaluate the entire resolution of the action based on which powers are currently in play.