Ha, it's thundering here, so I will write a little more.
I need to reword the death comment--didn't intend to imply that there's anything after death, just that it's the only complete end of both pleasure and pain. Until then, one or both of those is always present.
On happiness, I see I made an error-- I meant to say maximum happiness was a pleasurable feeling resulting from maximizing pleasure in all areas of life-- basically, it is pleasure of having the full cup of pleasures, or at least more pleasure than not, especially in regards to the life time "movie" of that cup. Without including maximum, I've inadvertently left out more ordinary situations. By saying it is a pleasurable feeling, I do not believe I have left room for it to be other than pleasure, and I'm not sure how that could be derived. We have all sorts of different sources of the pleasure feeling, and the response to awareness that our life's cup is mostly full of pleasure is itself a pleasure. I will say that at least that's what I mean by happiness. The feeling is the same as for other pleasures, but the stimulus is specific.
Maybe I could just reword it as "happiness is the feeling of pleasure that comes from awareness that one's lifetime cup is more full of pleasure than pain."
On absence of pain/fullness of pleasure, I am baffled by your answer-- it seems like you are disagreeing with the synonymous aspect, although you say you aren't, and I can't get a grasp on your train of thought. I would tell the person who says minimizing pain results in maximum pleasure that he is correct, and then I'd ask how he plans go go about it. If it is by withdrawing from activity, and he wouldn't listen to my advice that this will result in more pain, not less, then I'd tell him to check in with me in a year, earlier when he gets bored.
If he asked me how to minimize pain, of course it depends on the situation he is in, but I would give him ways to maximize pleasure, and tell him it is the exact same end result, and IMO, _easier_ to approach actively by seeking pleasure than by only withdrawing from pain. But if the withdrawing did result in minimum pain, it would have worked just as well. There should be no difference at all.
I'm not even trying here to describe what pleasure feels like, if that's the issue--that's like trying to describe "sweet." If they don't know the feeling of pleasure by adulthood, I can't imagine I would be able to help them out. There isn't any sufficient description, IMO, other than by using synonyms.
On the size of the cup... I guess this is just me seeing the metaphor differently. I don't see the cup as representing numbers of experiences but the organism's feeling capacity. I don't think it helps to imagine people's cups of different sizes, if the cup represents feelings. Because then you'd have to say, well your cup might be full of pleasure but it's too small, so you aren't having enough. And how would you know? How would you measure it, if they are saying nope, I'm in total bliss here-- how can you say they could have more, based on an outside assessment of their activities? If they aren't having enough pleasure, their cup isn't full, rather than being too small-- they still have pain.
Instead of restricted experience shrinking the cup of feeling, I would see it as the cup being in mostly a pain state, perhaps of low intensity boredom.