Godfrey to continue on this part of the discussion in an admittedly narrow and technical way rather than making a super important point, I wanted to make another comment on your statement here:
So, interestingly, the maximum pleasure of the entire organism is the absence of pain, whereas the absence of pain is the minimum of pleasure for any specific location in the organism.
I made a comment on this already, but here is something more about considering any interpretation of "absence of pain" as a "minimum":
From section IV of Book Two (again I am rewording to make it clear who is speaking so check the original:
Torquatus: ‘Can then anything be sweeter than to feel no pain?’
CIcero: "Nay, be it granted that there is nothing better, for I am not yet investigating that question; does it therefore follow that painlessness, so to call it, is identical with pleasure?’
Torquatus: ‘It is quite identical, and is the greatest possible, and no pleasure can be greater.’
Presuming that last sentence is translated correctly, and I have no reason to doubt it, this is another explicit statement that the Epicurean position is that:
1 - "Painlessness"/ "Absence of Pain" is IDENTICAL to pleasure. I interpret that to mean that the two concepts ("absence of pain" and "pleasure") are two separate words being used to describe exactly the same thing, meaning that the two words can be used interchangeably in referring to an individual discrete feeling.
2 - That using the term 'painlessness" or "pleasure" without any modifiers or caveats can also imply that you are referring to pleasure at the "macro" or "whole organism" level and therefore you are referring to "pure pleasure" which by definition means the "greatest pleasure," "no pleasure being greater."
My point in this post is that i am cautious in the wording so we can try to track the Epicurean usage. If you refer to a person, or a person refers to themselves at the moment, as being "painless," then that seems to be the equivalent of saying that they are at the height of pleasure in both body and mind as a full organism. If you refer, on the other hand, to some part of your body as being painless (such as hand or foot or your mind) then you are just referring to a location and saying nothing about the rest of the body or mind, which could be experiencing pain at the same time.
So I am cautious about referring to "painless" as a starting point. A painless foot and hand and adding them up is a good start, yes, but "painlessness" at the macro full organism level is apparently being defined as being 100% full of pleasure and a very great pleasure. Now maybe "variation" can explain this in a way consistent with "starting point," but I'm not sure that's consistent with the texts.
Maybe in the discussion of variation we'll find more to go on as to that aspect.