Your points are good, and make me question the wisdom of seeking to explain the evolution of ethics in terms of prevailing societies or cultures altogether. (In fact there are paths leading away from that approach that go into some of the darker and murkier aspects of sociology that I would just as soon avoid.)

Perhaps a more useful idea is that of paradigms. After all, a paradigm shift doesn't necessarily mean that a culture or society has really changed. France before and after the revolution, for example, may not have been all that different of a society, even if its heirarchy became more ethical as a result.