Ok, let's run through a few brief points. Again, I'd much rather do this over a pint. I'll focus on relativism. So:
1.
It may well be the case that we all agree Hitler committed a great many crimes, but all of them were entirely consistent with his own ethical code . . The assumption that the same code of ethics bound our leaders and the Nazis is risible, each followed their own judgment.
Two distinctions: firstly, between consistency and truth; secondly, between motivation and truth.
i. On the first, it is trivially true that there can be two equally consistent, but mutually exclusive (and mutually exhaustive of the territory), sets of beliefs. This being so, one of them must be wrong. Noting that Hitler's ethical code was consistent (which I don't think it was, but let's assume it for the sake of argument) doesn't entail anything about its truth.
ii. On the second, please notice that it is one thing to be motivated by a set of beliefs, and another thing altogether for those beliefs to be true. Take, for instance, the laws of logic, or mathematics. P & 'p implies q', taken together, imply that q. If you 'follow your own judgement' when you're engaging in a piece of deductive reasoning, and you happen to be irrational, then sure - you're following your own judgement, but you are still bound by the laws. That's why we can say that people make mistakes in reasoning - because there's a standard against which we can appraise their action or reasoning, regardless of what their motivations were.
2.
Barring some standard sent from the Divine, or some transcendental inspiration, how can you maintain that human ethics are either constrained by or tendent to a universal standard.
In short, because human ethics result from our status as dependent, rational animals. That's what provides the universal standard - since all people, minimally, fulfil this criterion - and it's also what entails that ethics is categorical, i.e. binding on all people. But that's a thesis-length argument; I'm not going to go into detail. At least a pint-length argument, anyway.
3.
you surely cannot be so blinkered as to fail to notice how hugely divergent the world around you is from the notion you suggest
What notion do I suggest? I only invoked professional philosophers in order to discredit the suggestion that moral realism / constructivism is a completely ridiculous position. I don't think it shows anything other than this.
4. If ethical relativism is necessarily true, it's not obviously so. Both of the 'arguments' which you adduced conflate [again] what people believe, with what is true. Sure, different people hold different beliefs; sure, I'm willing to grant that the reason people hold most of their beliefs is, first and foremost, because that's what they've been taught [or something similar]. But consider an analogy with science: most of the scientific beliefs which I hold, I hold because I've read something in a book, or been taught stuff in school, etc. Scientific codes [as it were] of different societies exhibit variation. So yes, scientific beliefs may depend, largely, on prevailing societal views. It doesn't mean that my scientific beliefs are false, and it certainly doesn't mean that all scientific beliefs are equally valid. For instance: plenty of people, up until relatively recently, thought that the Earth was flat, the Sun went around the Earth, etc. Witch doctors in Africa think that you can cure diseases with the ashes of burnt skulls. This doesn't mean that 'scientific relativism' is true; the flat-earthers were wrong, and we're most likely right.
The 'argument from cultural relativism' [the one which starts by noting that different cultures have different ethical codes] is discussed in Mackie's 'Ethics: Inventing Right and Wrong', and has a more detailed exposition in Williams' 'Ethics & The Limits of Philosophy'. It has also been widely discussed; in, for instance, Darwall, Railton, & Gibbard [eds] 'Moral Discourse and Moral Practice'. It certainly doesn't prove anything; it's doesn't show that ethical relativism is necessarily true (and how can something be necessarily true for a practical reason, anyway?). Sure, some professional meta-ethicists are relativists, but it's nowhere near as obviously true as you seem to think.
And yes, ex hypothesi, a significant proportion of the planet is pretty immoral. Is that so surprising? Maybe you haven't noticed, but even in Britain (which is pretty tolerant and liberal as far as the world's population goes) there's a high incidence of racism and homophobia, coupled with a pretty abject lack of charity. Plus there's war, rape, murder, sexism, domestic abuse, and so on, and so forth. But I think that the most widespread form of immorality consists of sitting around, playing golf, or drinking Burgundy, while children elsewhere die - preventably - from cholera, dysentry, etc.
note: I never claimed to be moral
like Juvenal said, difficilam est satiram non scribere.
