Godfrey I agree with all your conclusions in Post 16, but I also have to say that ....
Simply put, to my understanding the longer life of pleasure is more pleasant than the shorter life of pleasure.
... with which I also agree, would appear to most people to contradict PD19 (PD19. Infinite time contains no greater pleasure than limited time, if one measures, by reason, the limits of pleasure) unless we clearly explain why that contradiction does not exist in a way that normal people can grasp.
I think there are ways to do that, but those ways are going to -- as you say and as I agree - make clear that living a longer life of pleasure is better than a shorter life of pleasure, and that's going to conflict with a lot of modern orthodox interpretation. He who isn't satisfied with enough will never be satisfied! ... It doesn't matter to me if I die today! And all that....
Before I go further I looked back at Torquatus' initial presentation of the ethics and this is really the only part I see that touches on PD19 at all:
Quote from On Ends Book One[38] Therefore Epicurus refused to allow that there is any middle term between pain and pleasure; what was thought by some to be a middle term, the absence of all pain, was not only itself pleasure, but the highest pleasure possible. Surely any one who is conscious of his own condition must needs be either in a state of pleasure or in a state of pain. Epicurus thinks that the highest degree of pleasure is defined by the removal of all pain, so that pleasure may afterwards exhibit diversities and differences but is incapable of increase or extension.
I think we can productively ask why Torquatus's summary of Epicurean ethics *apparently* does not contain more explanation of this -- or maybe it does and we are just not seeing it. Rather than concluding that Cicero stacked the table and just omitted the explanation, I think we can infer that Cicero's interpretation of PD19 as meaning that time doesn't matter - which is probably the interpretation that prevails in Epicurean circles today -- is where the error lies.
Torquatus never says that time doesn't matter, and the common senses position is that time DOES matter. Maybe the (limited) point being made is that the experience doesn't get any "better" -- but that word "better" is where the devil resides in the details. I think we should look to the argument people seem to be making about virtue being complete in itself for a clue as to how pleasure can be complete in itself.
It would be perverse to interpret Epicurus as saying that it doesn't "matter" to us how long we live, and yet that interpretation prevails.
It seems clear that "the highest degree of pleasure" as stated by Torquatus in interpreting the "no greater degree" in PD19 is being given a limited technical meaning that is absolutely not intended to wipe out a common sense understanding that a longer life of pleasure is generally going to be preferred to a shorter life of pleasure.