I feel I identify more strongly with your synopsis than with atgatg's. In particular it dispenses with the replacement of "God" by "nature" as an object of "worship" (a very Carl Sagan thing to do), and gets straight to the point: that no ethical system is "provable", but that many are possible based on different choices of axioms, and that you are now going to set out what you perceive as the irreducible choices you are making, and follow these through to their consequences. I also agree on your particular choices of axioms (esp. the first five).
It may be that atgatg is more in alignment with you than one might suppose, given some comments made in the entry you linked about how these things are indeed irrational and the product of social mores.
So then, we can judge what is good or bad partly according to the values of our upbringing and of the society in which we live. But then what is to help us if the society we live in is depraved? -- and on what basis could we possibly make such a judgment? Ultimately it must be that the social good comes back to the good of the individual, for if one attributes any form of good to societies in their own right, individual unhappiness is certain to result. (This point of view presupposes that individual happiness is "The Good", but I personally like that assumption.)
And yet I realize now: if we attach no significance, none of "The Good" to society as a whole, collective anarchy is sure to result. Ah, that koan that Lord Russell posed for us:
Philosophers have been divided into those who wished to tighten social bonds and those who wished to relax them. With this difference others have been associated. The disciplinarians have advocated some system of dogma, either old or new, and have therefore been compelled to be, in a greater or lesser degree, hostile to science, since their dogmas could not be proved empirically. They have almost invariably taught that happiness is not the good, but that "nobility" or "heroism" is to be preferred. They have had a sympathy with the irrational parts of human nature, since they have felt reason to be inimical to social cohesion. The libertarians, on the other hand, with the exception of the extreme anarchists, have tended to be scientific, utilitarian, rationalistic, hostile to violent passion, and enemies of all the more profound forms of religion. This conflict existed in Greece before the rise of what we recognize as philosophy, and is already quite explicit in the earliest Greek thought. In changing forms, it has persisted down to the present day, and no doubt will persist for many ages to come.
It is clear that each party to this dispute -- as to all that persist through long periods of time -- is partly right, and partly wrong. Social cohesion is a necessity, and mankind has never yet succeeded in enforcing cohesion by merely rational arguments. Every community is exposed to two opposite dangers: ossification through too much discipline and reverence for tradition, on the one hand; on the other hand, dissolution, or subjection to foreign conquest, through the growth of an individualism and personal independence that makes co-operation impossible. In general, important civilizations start with a rigid and superstitious system, gradually relaxed, and leading, at a certain stage, to a period of brilliant genius, while the good of the old tradition remains and the evil inherent in its dissolution has not yet developed. But as the evil unfolds, it leads to anarchy, thence, inevitably, to a new tyranny, producing a new synthesis secured by a new system of dogma.
Bugger all this for a lark. I need to take a walk or something.