I don't like the word concepts here. Pain and pleasure are visceral felt sensations and are experienced by all creatures. Granted, we have to give words to them whether that is pain/pleasure, algos/hedone, dolor/voluptas. But they're not, in the end, abstract or constructed concepts like emotions.
At least for now, I think we're going to have to agree to disagree here, although this paragraph I quoted really doesn't express the point in issue. I of course agree with your view of please and pain and feelings. I just also believe that the same words can also be used as concepts to denote the full spectrum of pleasurable feelings (same with pain), and I think that Epicurus is using it both ways in different contexts as needed.
For example i think references to "limit of pleasure" are conceptual. Of course we can prove our concept is accurate by looking to the feelings, and that's why it all makes sense. But the "limit of quantity of pleasure" does not in my mind describe a "particular feeling." I would say that it describes a conceptual total that differs in every way between individuals other than in the conceptual way that it excludes all pains. I see the word "happiness" much the same way - it is certainly possible to "feel happy" but judging a life to be "happy" or "not happy" is mainly a conceptual categorization.