Cassius I'll defer to your point about it being appropriate for Epicureans to define the telos in terms of "happiness" if that term is being used in the correct Epicurean sense. I just don't want to see it used as a way to downplay ἡδονή. I have the impression that Gassendi did a bit of this--though I'll have to go back and look at his essays again.
As you say, "happiness" sometimes denotes a feeling, and a feeling can't be untrue/unreal. Even Pinsof acknowledges that happiness feels great.
Regarding happiness as a concept, though, wouldn't the validity depend on the criteria? If someone invents a type of εὐδαιμονία that is not founded on sensations, feelings, or prolepsis, then the concept is empty opinion and thus illusory. Since the majority of people are not Epicureans, would it be likely that many if not most people use incorrect criteria, and thus empty opinion/illusion prevails?
I have the sense that we are "agreeing fiercely," so this is all just to clarify/correct my earlier post.