"Well, then, where is the injustice of man's Fall?"
I guess I don't understand the question. Are you asking if the fall of man was an injustice according to God's plan?
If anything happens contrary to God's will, it is unjust (by definition). The way Protestants see it, man's Fall, then, was not (could not be) against His will and therefore is not an injustice to God that requires justice.