it seems as if “absence of pain” as a concept denoting the limit of pleasure is primarily intended as proof that a life of consistent pleasure is possible and attainable. If pleasure had no limit, then we’d constantly be trying to fill our bottomless cup.
Since that is the analogy that is explicitly stated at the opening of Lucretius Book 6, I think you are on firm ground:
he then did understand that it was the vessel itself which wrought the disease, and that by its disease all things were corrupted within, whatsoever came into it gathered from without, yea even blessings; in part because he saw that it was leaking and full of holes, so that by no means could it ever be filled; in part because he perceived that it tainted as with a foul savor all things within it, which it had taken in.
also:
As I’m writing this it doesn’t seem as clear to me as it did in my head… This seemingly simple topic makes me head spin!
What you wrote is very clear and makes complete sense. When you see that the vessel has a limit then you see that it can be filled. The problem comes in trying to stretch the analogy too far. Analogizing an Epicurean-inspired life to a well-formed jar solves the false allegation that a life of pleasure can never be complete, but it does not answer the question of what kind of pleasure you should use to fill your jar.
If any pleasure would do, what about the pleasure of thinking that god directs your life so that everything works together for good if you love the lord, or the pleasure of thinking that you will be lifted to heaven when you die? It is undeniable that those thoughts can be pleasurable too -- are you going to fill your jar with those kinds of pleasures?
The answer is "no," and while the vessel analogy can still be used to an extent (maybe you can say that those pleasures evaporate so the jar doesn't stay full) the vessel analogy can't fulfill every need for explanation of the issues.
This wouldn't be a problem except for those who want to suggest that "absence of pain" is a specific pleasure in itself. It is impossible to name such a pleasure, because all pleasures and pains are discrete experiences, and "absence of anything" doesn't describe a specific experience. Absence of a heap of sand doesn't tell you anything about what IS at the location you are discussing. Absence of pain tells you that pleasure is there, but ONLY because you have previously identified that whenever pain is absent, what is in that location is some form of pleasure. But you aren't stating what kind of pleasure is present without going into further detail, and if anyone wants to suggest that all pleasures are exactly the same then they are speaking nonsense. That's ust like Epicurus describes (in the letter to Menoeceus) the man who says that it is better to never be born, or rush to death. Such a man is talking lies or nonsense, because he could easily end is life if that is what he really thought.