"The confusion stems from the (perhaps deliberate) parallel construction, by which the sum total of pleasures is related to the sum total of the sentient human."
I think that's a particularly important observation. I'm not able to validate that it is true from the Greek, but I think that the description of what Epicurus is doing is accurate. It looks to me like Epicurus is definitely evaluating "the sum total of pleasures" in relation to "the sum total of the sentient human."
To me, the comparison of the "sum total of pleasures" to the "sum total of the sentient human" is the "vessel" analogy. Just as the example is given in the opening of Lucretius Book 6, you can view a human life as a vessel (jar / vase / whatever) and realize that a vessel or a life can only be filled so far with pleasures.
After you pour in pleasures and fill the vessel of life to the rim, you can vary the pleasures by pouring in more pleasures, but some of the existing pleasures will overflow over the rim (the same quantity that you pour in will be expelled). The vessel can never get "more full."
When the "sum total of the sentient being" is full of pleasure, it is full of pleasure, and you can never be more full than full no matter how much time you spend pouring in new pleasures.