I'm glad my interpretation interested you. I'm glad also that you hope to read the whole dialogue again. Perhaps both of us can hope that you'll come up with some further insights.
If you're going to re-read the whole, let me suggest my interpretation of the overall structure. Basically, Plato plots the dialogue so as to generate constantly increasing dramatic tension. We start off with Gorgias, who praises rhetoric. But he is caught in a contradiction by Socrates. Next comes Polus, who correctly shows that Socrates' refutation of Gorgias actually fails because Gorgias could have avoided the contradiction. Polus then describes rhetoric and politics, avoiding Gorgias' needless mistake. But Socrates catches Polus in contradiction (at the point you and I have been discussing), so Polus' views apparently are refuted. But Callicles then takes over and correctly shows that Socrates' refutation of Polus actually fails. And Callicles then ventures a description of politics that avoids Polus' needless mistake. So the dramatic tension builds and builds; Socrates once again has to start all over to win his point. As Plato portrays it, Socrates then finally does refute Callicles. (On my own reading, Socrates' refutation of Callicles doesn't really succeed; and it actually is in the Republic that Plato bids to find his remedy.)
The Gorgias is perhaps the most dramatic of Plato's dialogues. But if so, it also is the most artificial. For, to realize the intricate plotting described above, Plato must repeatedly put into Socrates' mouth arguments that Plato knows to be fallacious, but that are presented persuasively enough that Socrates' interlocutors of the moment can plausibly fail to see the fallacies. Tour de force!
Socrates follows the arguments of the others, while they turn the discussion according to beliefs they keep, without knowing exactly what they mean. It's not my impression that Socrates invents fallacious arguments. A fundamental existence of a conventional good is impossible, since Plato believes in The Good and recognises Good at the roots of World creation and Gods. This is what is manifest by the final section of the dialogue and the reference to a Last Judgement. All that Socrates/Plato tries to do in Gorgias, as I read it, is to show how being good responds to this metaphysical destiny of the world, beyond social conventionality, and that this response is to be discovered by the man who seeks truth and not ways to support even the very opposite cases according to interests of the time, which is the sophists' rhetorical art.
1. My claim was that Plato invents fallacious arguments. That's obviously so, for everything that appears in Plato's dialogues - including those arguments that Plato portrays as refuted (e.g., Polus' arguments, Callicles' arguments) - was authored by Plato.
2. You're free to read Plato any way you wish. I suggest, though, that you might find it satisfying to attend more closely to what he actually says. Here in the Gorgias, Plato presents a series of arguments as to whether one should be good. I think Plato hoped his readers would focus on identifying and evaluating the arguments presented there. I think he crafted the dialogue so that readers who took that approach would find the experience satisfying. You claim that "Plato believes in The Good and recognizes Good at the roots of World creation and Gods". That may or may not be so, but there's no hint in the Gorgias text that "the metaphysical destiny of the world" is in play there. I think you'll find it more satisfying to save up your pronouncements about The Good and World creation until you're reading other dialogues like the Republic and the Timaeus and, when reading the Gorgias, to focus on the arguments Plato took the trouble to present there. That is, I think, what Plato intended.
Xenophon, in his "Memoires of Socrates", uses a phrase - 'kalokagathia' - to refer to the virtue he reports Socrates as attempting to teach, according to Robin Waterfield (in an introduction in his Penguin Books compilation of Xenophon's Socratic writings). Waterfield translates Xenophon's 'kalokagathia' as 'true goodness'. Waterfield says 'kalokagathia' appears first in Xenophon's writings, and is related to a common Greek phrase for 'truly good' - 'kalos kagathos'. According to Waterfield, 'kalos' refers to physical attractivness, but also to moral goodness and the appropriateness of a thing in its setting. 'Agathos' does not, in Xenophon's time, refer to external qualities, but has the same extension as the English 'good', used for everything from moral goodness ("He's a good man.") to utilitarian value ("That's a good knife.").
So far as I can understand these matters, Waterfield's comments seem entirely to support George's interpretation that Socrates in the Gorgias seems to discuss all goods together as 'kala kai agatha'. If so, then (per the revised interpretation I adopted May 22) Socrates' argument in this discussion with Polus would indeed have to turn on differences between nature and convention rather than on any purported difference between 'kalos' and 'agathos'.
Werner Jaeger was a German Classicist who wrote a masterful three volume on "Paideia". In there he describes Kaloskagathos as "the chivalrous ideal of the complete human personality, harmonious in mind and body, foursquare in battle and speech, song and action". It literally means 'the Beautifull and the Good'. It was an ideal that Homeric Greek culture strove to obtain. Being a kalos kagathos, became the focus of education and to this ideal, the whole human being was conditioned and trained to become.