These are great questions! Let me go through them and give you what I think is the answer. Not everyone is going to agree with me, but I think what I am about to say is the clear implication of what Torquatus was explaining to Cicero:
I think we better start here:
1) There is only pleasure and pain.
This is not so simple that we can stop without explanation, and failure to clarify it is a source of much problem. Yes, Epicurus says that nature gives us only pleasure and pain as feelings by which to know what to choose and what to avoid.
But you are asking a series of questions about "Pleasure" and "Pain" with capital "p's" -- You are asking about the concept of pleasure and the concept of pain. As a concept which serves as a stand-in for the "goal of life," "Pleasure" is a conceptual term which encompasses all possible experiences of pleasure, from the longest and most intense to the shortest and least intense. All pleasures are conceptually part of "Pleasure," but all pleasures are not by any means identical. The same thing goes for pain.
A large part of the problem in general discussions of Epicurus is that people are talking about "Pleasure" as the conceptual goal of life without making clear that the goal of a real person's real life is not a "concept," but a set of real experiences that cannot be described completely in the term "Pleasure" any more than a map constitutes every detail of an area of land that is being mapped.
And not separating those two contexts leads to most of your questions:
3) Let’s say my hunger and thirst are satiated, my body is healthy, I have good friends, and I fear neither gods nor death.
4) This being the case, I am experiencing the absence of pain/fullness of pleasure, am I not?
The answer to (3) is "not necessarily." We are philosophers, and you have not stated in 3 that you are not suffering any pain. Torquatus' examples, including the comparison of the host pouring wine and the guest drinking in, are stated in the context that the example includes as a premise that they are otherwise without pain. Anyone who is "without pain" is therefore definitionally and conceptually at the height of pleasure, because you are speaking in broad definitional terms. Pure pleasure - 100% pleasure - cannot be made more pleasurable by removing impurities, because "pure" means without impurity, and 100% means a mathematical limit for any given subject.
So I would say that your conclusion in (4) is not properly established by (3). you have listed a number of pleasurable conditions, but you have not by so doing confirmed that your "jar is full" and that there is not more room for more pleasure in your life.
Do you agree that the conditions listed in point 3 are all that is necessary to experience the absence of pain? If so, and if that is the limit of pleasure, why do you also press that these things are not enough, and that Epicurus also encourages these “active pleasures” like playing and dancing?
So the answer here is that I do not agree that the points listed in 3 are "all that is necessary to experience the absence of pain" in total. They could be if you also stated that the person was without pain, but unless someone is affirmatively stating that the person is "without pain" then you don't know.
This would apply to the Chrysippus hand challenge. We know that the hand was at the "height of pleasure" only because the hypothetical was that the hand was "in its normal condition" and not in pain. Could any particular hand get more pleasure from a warm massage rather than in its normal condition? I think the answer is clearly yes, but that doesn't mean that the point made by Torquatus is incorrect. "Pure" pleasure does not necessarily equate to "most intense," or "longest duration" or "largest part of the body affected." Torquatus did not say that the hand was experiencing any of those -- not the most intense please, nor the longest, or the most extensive. The debate about the hand was in terms of the "height," or as in PD03, the "limit of quantity." These are technical terms suitable for philosophical debate, but they don't tell you the difference between good heath and a good massage.
If some pleasures are more pleasurable than others, wouldn’t that make my jar “more full”?
Per PD09, pleasures differ from each other in at least the qualities of intensity, duration, and part of the body affected. Some particular pleasures ARE more intense, or last longer, or involve different parts of the body, and only an idiot would deny that. But all pleasures are unified in being feelings that we find desirable, and thus one of them is not more conceptually "pleasure" than is another.
This is Pleasure with a capital "P" - conceptual pleasure - all of which carries the same definition of a desirable feeling. The concept never changes, even though the particulars can and do change.
If some pleasures are more pleasurable than others, wouldn’t that make my jar “more full”? How does this fit together with absence of pain being the limit of pleasure? And if the jar can be full while containing different levels of pleasure, then what is it even measuring?
A jar which can contain only "Pleasure" and "Pain" cannot be defined as full containing different levels of "Pleasure," for reasons that are obvious - we are defining the possibilities and there are no options outside our hypothetical. But different jars of "Pleasure" can and certainly will contain very different mixtures of difference types of pleasurable experiences.
We all know this to be true, but what you're asking is the right question. How can different jars be other than the same if they all are full of "Pleasure?" And the answer is that pleasures are not "just" concepts. Pleasurable experiences are what is real, while "Pleasure" is a concept that philosophers use in debate. The same goes for "Happiness." The wise man can by "Happy" even which tortured on the wrack or in the throws of dying from kidney disease, because "happiness" is a concept we can define as an overall assessment of more reason for joy than for vexation, while "a feeling of happiness" is not what is generated by torture machines or kidney stones.
I’m playing devil’s advocate a little here in order to understand the logic. Again, I agree with the conclusions. But I’m having trouble seeing how it all fits together. It feels almost a little contradictory.
Everyone ought to be asking these questions, because unless you demand consistency and clear answers, no one ever gets anywhere. And far too many people outside this forum are ignoring these issues and thinking that they can wink and smile and fool others - and themselves - into thinking that Buddhist nothingness / Stoic apathy really is super pleasurable.
And that's in my view why Epicurean philosophy has been stuck in the mud ever since the last of the ancient Epicureans passed away.
No one except a confirmed Buddhist or Stoic or Sadist really believes that "absence of pain" understood as 99% of the world understands it is really worth being a goal in life. But the majority of Epicureans have fled from the idea that "absence of pain" really means "pleasure" because that would not be respectable, or virtuous, and to say so would earn them the disapproving frowns of the intelligentsia.
In my view, you can either demand consistency and clarity, in which case you come around to seeing that these are definitional and philosophical issues. Once you accept that, "absence of pain" becomes nothing more than technical terminology for exactly the same thing expressed by the word "pleasure."
The reason you've chosen technical terminology like that is because you are philosopher, and you're dealing with technical objections from the Platonists and others who demand to know "the limit" of pleasure. Absence of pain is highly useful for answering that question - for identifying the theoretical limit.
But "absence of pain" in this context is conceptual, and this conceptual answer does not tell you whether to stop when you're not thirsty or hungry. You have to apply also the rest of the conceptual framework, in which you're previously identified that all pleasure is desirable, and that there would never be any reason whatsoever ---but one -- not to seek to obtain all the pleasure you can. And that single reason not to pursue a particular pleasure is that you evaluate that pursuing that pleasure would result - in the end - with bringing you more pain than pleasure.
I suppose I should address too the related question of how long you wish to live, or how much pleasure you wish to experience while you are alive. To me, the answer Epicurus points to is that "satisfaction" comes from realizing the limit that you are human and mortal and that nature allows you to live and pursue pleasure for only a certain period of time in good heath. You don't need to be king or the most famous person in the world to consider your jar of life to be full of pleasure. But if you have consciously avoided, through fear or otherwise, stepping up to experience the pleasures that are possible to you, then the reasonable and thoughtful person is going to naturally feel regret at passing over pleasure for no good reason. And "regret" is a pain.
Edit: As always, I'm not Epicurus and can't speak for him. These answers are just the best I can do today given my state of analysis and reading from all the various materials.