Anonymous, you have to keep in mind that God is infinitely just and that, for all His infinite power, there are certain things where He is powerless. To name a few: He cannot lie to himself (or to us), he cannot stop being God, and He cannot contradict Himself.
Another thing you have to keep in mind when contemplating the mystery of the original sin, and of sin in general, is precisely that it’s a mystery (misterium iniquitatis). That’s another thing God cannot do: He can’t reveal all mysteries. Or, said it in a different way, God without mystery would not be really God, unless we’re willing to accept the irrational contradiction of an infinite being contained (comprehended) within the boundaries of a finite mind.
And last, but not least, keep in mind that even though we are all created by God, Adam and Eve’s act of creation is of a different kind. When God created Adam and Eve, His creation act, besides being out of nothing, was twofold: he created both the individual human and its human nature. When He creates us, he only creates the individual; the human nature we inherit from our parents, who inherited from theirs, and so on, until reaching the first human couple.
I think the right question would be why did Adam and Eve have the “power” to affect human nature. The answer to this question (if it can be called answer at all), I haven’t found it, but I have a theory. I’m not certain this is orthodox catholic doctrine, but it (kind of) makes sense to me. I believe they had that power because they possessed human nature in its totality. We share in a human nature with billions of other individuals, so our personal sins cannot affect it, unless we coordinate with the rest of humankind to commit the same sin. When Adam and Eve sinned, humankind sinned. Under this light, it’s understandable the concern of the Church with the theory of evolution: if we don’t descend from one single couple that lived and sinned in a time when nobody else lived, all the framework of our faith crumbles.
And one more thing: don’t let yourself be fooled by the simplicity inherent to the allegorical nature of the Genesis tale. First of all, the magnitud of a sin can’t be judge independently from the circumstances of the sinner, in this case somebody without the inclination of the nature that we must nowadays overcome, and many other preternatural gifts that we don’t possess. Sin is always something terrible, probably absurd from a divine standpoint, and in their case even more so considering that they lived in a state of infinite bliss. It wasn’t the reaction of somebody overwhelmed by their own weaknesses against the seeming arbitrariness of an incomprehensible ruler. Theirs was an unjustified act of rebellion, of disobedience against somebody whom they perfectly well knew only deserved their eternal gratitude. Many centuries later, the “obedience to the point of death, and death on a cross” of another man that would replace Adam and Eve in their primacy, was needed to undo, to cancel out the first disobedience. The contemplation alone of all the sufferings of that Man that was also God so His sacrifice would have the infinite power to erase an infinite offense and an infinite number of other offenses, should serve as a clue to the gravity of that original sin, that incomprehensible “misterium iniquitatis.”
And as for them not knowing good from evil, that’s in my understanding an error in your interpretation. I believe they knew perfectly well the evil they were doing; their faculties were definitely clearer than ours, which are clouded precisely by the wound inflicted by them on our human nature. The devil’s temptation was not to understand from a human perspective, but to understand as God understands. The temptation was to understand in the same way of a God whose knowledge alone had not only creative power, but also the power to make things good or bad. It was a temptation to usurp the place of their own Creator and Supreme ruler. And it’s, by the way, the greatest temptation of modern man, to become autonomous from God, to be to himself his only ruler, to claim the power to decide what is good and what is evil.