I don't think what you're implying is correct, but I can't find it in the compendium and I'm not at home to look at my books. I was pretty sure that "order" has to do with the entries of a power such as "Hit", "Miss", "Effect", and "Special", but within an entry the "order" didn't matter or was irrelevant.
The written text of a line in the power is executed in order. Entries in a power are executed in order, that as much implies the micro-order within the entries themselves as well as the macro-order of the entries' placement.
The specific text used in the PHB2 is 'The order of information in the power.'
The text of a specific entry is information in the power, and thus its order is significant.
You're in the step where you are evaluating Hit or Miss after the target is knocked prone. The only requirements are (as quoted above) that you hit and you have CA. I don't see any verbiage that says you are required to have combat advantage before you ROLL to hit.
No one is claiming you must. You have to have combat advantage
when you roll to hit. But: this power provides no method for garnering combat advantage during the roll, ergo, you can only logically have combat advantage before, or after the roll. After the roll will not affect the roll and therefore will not affect the hit. Therefore, the only possibility is that you must have the combat advantage before the roll.
This power rolls its attack first, as you pointed out, the Entries of a power are enacted in order.
You execute the Attack line, then the Effect line, and finally, the Hit line. The Hit line does not tell you when you roll your attack, it tells you what to do if you hit. Similiarly, the Miss line does not tell you when you roll your attack, it tells you what to do if you miss. If these were true, then for every power with a Hit and a Miss line, you'd roll two attacks.
We all know -that- interpretation is bunco.
As well, the specific example tells you that when Effect lines come before attack information (and Attack: is part of the attack information), that is how they indicate a power where the effect occurs before the attack.
This particular power has:
Attack: number vs defense
Effect: knock prone
Hit: damage
At the point where you evaluate "Hit" you have CA and thus get sneak damage (unless as I said above the ordering is a typo).
Evaluating the Hit line and rolling a hit are not the same thing. This is a line that triggers if you hit with an attack. It is not the line that tells you -that- you've hit with an attack. The attack is rolled on the attack line. Resolving that hit can come after the determination of the hit. In fact, it kinda has to.
If it was not intended to work this way they could just as easily have said
Effect: Before the attack you knock the target prone
OR
Hit: damage
Effect: knock prone
OR
Hit: damage and knock prone
Miss: knock prone
And if it worked the way you stated, they could have mentioned it as well. Your argument does not present any sort of logic that advances your point.
For the first you would have CA for both the attack and damage portions of the power and for the other two you clearly would not have it for either. Since they used none of these the only possible interpretation left is as I described.
It does not matter, however, because Sneak Attack does not have multiple points at which it can trigger. It does not say 'when hit, and then you damage a target you have combat advantage against' which would allow you to do what you say. No. What it says is 'when you attack and hit a target you have combat advantage against.' It does not say 'When you resolve a hit against a target...' so you go to the definition of what a hit is; when you beat their defense on an attack roll.
The only point where you have beat a defense on an attack roll is when you make that attack roll. Therefore, that is the point at which you hit. The Hit: line only tells you how to resolve that hit.
To quote Sir Arthur Conan Doyle:
"When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth."
In order to have remains that are true, you must eliminate the impossible.
You have not done so. The attack line is when you make the attack roll, and you have not made this impossible. The hit is scored when you make an attack roll, and you have not made this impossible. See above where I show how CA must occur before the attack roll as an example of 'eliminating the impossible.' What you have done is an example of making assumptions, declaring anything else impossible by dint of them not matching your assumptions, and then saying that proves your assumptions to be true.
The former is deductive reasoning, the latter is circular reasoning.