I will say that when you wrote:
Epicurus could not simply state a goal of "more pleasure than pain" without justifying that pleasure was in fact the goal.
I have to say, in a small way, I disagree with you in emphasis. I think Epicurus DID justify pleasure as the goal in how he laid out the entire Canon, Physics, and Ethics. Lucretius does the same. It wasn't that Epicurus just "stated the goal" that pleasure was the goal. He built, from the ground up, a mighty fortress to defend that assertion with the flag of pleasure flying from the turrets! He continued to let fly arrows at his opponents through his writings until the Christian nuke ALMOST wiped him out. But eventually, "he" (as in his and his followers' works) could emerge from the underground bunker and begin some guerilla warfare.
Eugenios I was re-reading this and I realize that I don't think I understand your point in this passage. Are we both not saying exactly the same thing? The first priority and order of business of Epicurus was to establish that Pleasure is the goal of life, which the other Greek philosophers had disputed (and held otherwise in favor of wisdom, virtue, etc.)
So we agree that it was Epicurus' first priority to establish his reasoning as to why pleasure is the goal of life, and to point out the reasons why other philosophers' assertions were incorrect?