"I think the evidence that Epicurus believed a complete life can't get any better is clear; I don't know how else to interpret KD 3, 18, and 20-21, and Men. 128 and 131." To reiterate, I think there's strong evidence that Epicurus thought this; I personally don't know why he was so committed to it. But that doesn't change that, as far as we can tell, he was really committed to it. I don't necessarily disagree with your point about reaching an implausible interpretation; but, at least as things stand now, your approach seems to me to have more problems.
I always appreciate it when references to specific texts are given. It provides assurance that we're all looking at the same thing, although interpretation/translations sometimes/often differ.
In your assertion that "the evidence that Epicurus believed a complete life can't get any better is clear" you refer to those specific texts. Here's my take and a little of why I'm not entirely onboard with your interpretation.
To my understanding:
KD3 is Epicurus statement as to why pleasure CAN be the summum bonum against other schools' assertion that pleasure is inexhaustible or never able to be satisfied and therefore cannot be the telos. There IS a limit to pleasure, and it is the “The removal of all feeling of pain ... Wherever a pleasurable feeling is present, for as long as it is present, there is neither a feeling of pain nor a feeling of distress, nor both together.” (Inwood & Gerson (1994)) I think a key phrase there is "for as long as it is present." To me, this echoes Diogenes Laertius' "Two sorts of happiness (eudaimonia) can be conceived, the one the highest possible, such as the gods enjoy, which cannot be augmented, the other admitting addition and subtraction of pleasures." (10.121) I've always added "such as mortals enjoy" to the end in my mind; BUT Epicurus did say clearly that we can "live as a god among humans" (Men. 135) BUT we are not gods. That's also clear. Epicurus did not teach some kind of apotheosis from what I have read. We're still mortal. We still die. I will admit that there is a tension between living "as a god" and being mortal.
KD18 covers both physical and mental pleasures. Satisfying physical pleasures, per this KD, doesn't bring increase but only brings diversification/ variation / embellishment, depending on the translation of ποικίλλω. I like Epicurus' use of this word in that it evokes the various colors in a work of embroidery or other cunning workmanship. That's a poignant metaphorical connotation. The limit of mental pleasures in KD18 are specifically said to be due to reflecting on these bodily pleasures and sensations related to them and also having a eradicated "the greatest fears" to which I'm equating the fears of death, the gods, divine wrath: the first two KDs and VSs and first two lines of the Tetrapharmakos.
KD20 & 21: Again, this is Epicurus emphasizing how pleasure has a limit and why it can be the summum bonum. The "perfect way of life (I&G)/ complete life (Long) / complete way of life (S-A)" τὸν παντελῆ βίον (ton pantele bion) still to me evokes a life in which all goals are accomplished, in this case, the limits of pleasure have been achieved. But is it the understanding that pleasure is the telos along with the eradication of the fundamental fears of death and the gods that makes that "pantele" life or the actual elimination of all pain? I don't think Epicurus teaches that ALL pain can be eradicated from the life of the mortal human. He certainly couldn't do it, but he could do battle against physical pain and his greatest fears were eradicated, root and branch, from his mind. KD21 reinforces that what we REALLY need, at our most basic level, is limited. We can add variation of pleasures, but we don't NEED them. I think this is illustrated by Epicurus' periodic fasting and eating minimally so he KNEW what he needed to live pleasurably. So, if the going ever got tough, he knew exactly what was necessary for him.
I'm not going into Menoikeus since I think I've addressed that before. One other item I wanted to touch briefly on is:
Well, Epicurus's deathbed letter gives evidence that, in other cases of extreme pain, that pain won't interfere with blessedness. (Ditto for the, perhaps implausible, claim in Diogenes Laertius that the sage will be happy under torture.) If I didn't have this evidence, I'd predict that that pain would interfere with blessedness; and I worry that it does despite the claim that it doesn't. But I really believe Epicurus thought the pain didn't interfere, and the challenge is explaining why.
The problem people express with the "happy under torture" phrase (and it must be something Epicurus taught because it comes up in multiple sources) is that Epicurus, to my reading, never meant the sage would be joyous or giddy or happy in the common effervescent way we sometimes think of that. Diogenes Laertius uses "eudaimonia" that slippery, untranslatable, multivalent word. To me, and I admit this is likely idiosyncratic, the only reason I can think that a sage could have eudaimonia on the rack is if it means they could feel they did everything they could to have avoided this outcome. They would be in extreme pain, crying out, no question. If they were doing it to protect a friend, if they knew they had done everything to avoid it, they would still have an underlying sense of well-being and maybe that they had lived well and that the pain couldn't go on forever. They're not happy happy joy joy. But they have some perspective to give them some kind of "eudaimonia." And we're also talking a "sage" here, not your average student of the philosophy. Is some of this hyperbole to give the student confidence, to illustrate "here's the kind of mental fortitude you can get from following this path." Likely. But I'd rather take this idea of the sage rather than the Stoic sage who can't be concerned if his wife and children die in a war and who takes no more concern for a dead child than a broken cup.