You mention, "He had also previously stated, in PD3, that the limit in QUANTITY of pleasure is the absence of pain." Where is the word "quantity" mentioned? Magnitude, as stated in the quote, does not necessarily imply quantity. I tried to do some investigating, and the word we're contesting is "μεγέθους." Perseus.uchicago.edu defines this to be "greatness, magnitude, size, height, stature." This is very different from quantity in my opinion, and the alternative definitions show that "quantity" would be a bit of a mistranslation. Here's where I got the Greek-English principle doctrines: http://monadnock.net/epicurus/principal-doctrines.html
It is interesting you bring up the results of reading the quote explicitly. But I do not think taking them at face-value is misreading them. The language itself prevents the reader from construing them in any fashion other than what is written, unless we should say "Epicurus MEANT to write __." And that may be fine, perhaps in his lost writings he consolidated these views.
You mention, "Viewed from the perspective of quantity, any Epicurean who sees that pain has been reduced to zero also knows that it has been reduced to zero by filling the human experience completely with pleasure, so as to drive out all pain. In Cicero's phrase, 'Nothing is preferable to a life of tranquility crammed full of pleasures.'"
But should we think that pain can be driven out by pleasurable experience? We think of a man drinking away his sorrows, or someone going to a party to drive away his depression. Mental pain must be extinguished before we can experience ultimate pleasure in life, as ataraxia is the highest state of pleasure. Ataraxia is not achievable if there are underlying troubles in one's mind. Cicero's quote requires first, before the cramming full of pleasures, a "life of tranquility."
And again in Torquatus, there is this condition of the pleasurable life: "undisturbed by the presence ... of pain."
You mention, " ... before you choose to do something that entails a lot of effort, you better be sure that you will in fact get a lot of pleasure, and you better be sure that you do it in such a way as to minimize pain."
This is precisely why I argue deep friendships are impossible in Epicurean thought. Relationships require immense effort. However, often it is without the promise of a lot of pleasure. Often it is impossible to be sure one will receive anything at all in return. Even after weighing all the pleasure associated with the totality of one's friend/partner, the conclusion may be that the sacrifice outweighs the reward. The man who cuts off the friendship may be losing much more than the man who keeps the friendship and endures the suffering. As I worded it to Hiram, some rewards can only be received by those who do not endeavor for them.
To answer your last post - I've enjoyed all of Lucretius. It is probably my favorite work of philosophy. I would say that my favorite part of it is more to do with his arguments against the lamentation of death. Whether we wasted our lives, or lived full lives, the end of life should not be lamented. This was shocking to read, yet the arguments he made were very moving -- particularly when Lucretius directly addresses "the nature of things," and quotes her responses to the man lamenting his death.
Apart from his ideas of death, the arguments against immortality and the existence of gods were also very fascinating to read. The reasoning he uses is very simple and intuitive but impactful. One example: the soul is “begotten along and grows along with the body” (3.457); therefore, when the body deteriorates and dies, the soul does the same.
I was attracted to Epicurus through Lucretius, as I read his work before any of Epicurus' works. But Lucretius was a devout follower of Epicurus, and I was equally attracted by many of Epicurus' ideas once I began looking into them.
I think PD3 can be read in another way without disturbing the body of Epicurean thought. As per PD3, the limit of the magnitude of pleasure is freedom from pain. Let's assume there is no more pleasurable a state than ataraxia, total freedom of pain. Does this reduce the Epicurean to an ascetic, called to live a dull, boring life? I would say no, because of the following: is it possible for a human being to be happy with a totally empty, pleasure-free life? The absence of all pleasure surely should be the worst suffering one can endure! True "freedom from pain" IMPLIES the existence of a multitude of pleasures, both simple and complex.