Yep, I think we are going to have to disagree; however, let me try to win you over to the dark side... or at least try to fully explain my position.
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.
From my perspective, pleasure (hedone) and pain (various words: lype, algos) refer to the *feelings* we experience. However, I also agree with you that the words "denote the full spectrum of pleasurable feelings (same with pain)." As I read the texts, I understand Epicurus et al. to be saying that ALL our feelings - ALL of the ways that we experience what happens to us (the literal meaning of pathe) - fall into either being pleasure or pain. All we feel can be categorized into one or the other those overarching categories. So, I agree that he's "using it both ways in different contexts," but our feeling is the constant referent.
Diogenes Laertius: They affirm that there are two states of feeling, pleasure and pain, which arise in every animate being, and that the one is favourable and the other hostile to that being, and by their means choice and avoidance are determined.
I believe Epicurus and the ancient Epicureans were firmly rooted in physical reality. When you say "concepts" I hear "Platonic ideals that exist outside the physical world." And the word that he uses is consistent for pleasure - hedone - although different words for pain are sometimes used to juxtapose against pleasure.
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 agree that the "limit of pleasure" doesn't describe a "particular feeling." Instead, it refers to an animate being feeling only pleasurable sensations. Which, as finite, animate, mortal beings we can never achieve. It's a goal to move toward. That's why we choose and reject desires. To move closer to the limit of pleasure even if it is an unattainable goals for us mortal beings. Just because it's unattainable, it still is a worthy goal even if pleasures can be added and subtracted to our experience.
As it says:
Diogenes Laertius: Two sorts of happiness (eudaimonia) can be conceived, the one the highest possible*, such as the gods enjoy, which cannot be augmented, the other admitting addition and subtraction of pleasures (hedone).
Eudaimonia falls under pleasure. *"The highest possible" to me reads like "the limit of pleasure." We live as mortal beings and experience pleasure that can be augmented by adding and subtracting pleasures (pleasurable feelings).