I understand, as you say, it is "emphatically not" a requirement that an ethical/moral structure (sorry about the loose terms) make the same claim about what people should do independent of circumstances. Can this not be pointed to as the major failure of the philosophy of ethics?
There are a number of ways to approach this question, and I'm not sure if I can think of a coherent route which would follow any tight linear pattern (I've said it before, and I'll say it again now: I'm no metaethicist; these won't be my most graceful arguments!). So I apologize for just throwing a bunch of ideas out there in a somewhat haphazard manner (I'll try to preserve some sense of coherence!), but hopefully once they're all out there, it'll make sense why I think what I do about this issue.
First of all, I would want to point out that there are some actions which are, by necessity, always immoral. Murder is the example I always use (there may be more like it; I can't think of any at the moment..."violating" someone seems like it might work), because it seems automatically true that murder is wrong. But there is a reason for this. Murder actually means "wrongful killing." So it's wrong because if it weren't, it wouldn't be murder. The idea I want to highlight is that there are certain ways of describing our actions which are inherently value-laden. And by using these terms, we can often create the impression that we have come up with a coherent moral theory when we have really only applied our terms. So "murder" is automatically always wrong, but perhaps "killing" - what murder is when separated from any normative content - is not.
"Theft," to pull from Objectivist literature (for example, see this piece), is a word which only makes sense in the context of an institution of property rights. And even under the weakest sense of what it means to have a right to something, it must at least be prima facie wrong to steal (prima facie means "at first glance"). So it is no surprise that we generally agree that stealing is wrong (though many people might think that stealing is not always wrong, just like they would think that it's not always wrong to do something to someone else which they have a right against; see here, for example).
But when we separate out the normative content, "stealing" becomes simply "taking." And taking is not always wrong. I may permissibly take my towel to the beach, or take a drink of water at a public water fountain. And I don't think that's just because I'm playing word games and switching meanings: taking is not wrong by itself. Where taking becomes wrong is when we take something that we should not take - something that is either rightfully someone else's, or that is for some reason worthy of not being disturbed or removed from its original environment. But here the problem isn't simply the taking: it's the fact that the object I'm taking belongs right where it is. And to say that it "belongs" there requires that we come up with some exogenous conception of where stuff belongs (property rights being an example of this).
That seems like a reasonable lead-in to my second point. Once we are committed to describing our actions in value free terms, it seems to me that we need to make reference to circumstances in order to determine the moral character of our actions. For example, take the act of "killing a person." If I'm sitting in a coffee shop, and I see a stranger walk in and order a coffee, it seems like it would be wrong of me to kill her. But if I'm sitting in that same coffee shop, and I see that same stranger walk in, except now she's pointing a gun at me and (seemingly) getting ready to pull the trigger, we might not judge me so harshly if I were to kill her [Note: I won't get into the question of mens rea here, but I beseech my audience to give me the benefit of the doubt here. For any philosophers reading this: you know what I'm trying to say, darn it]. The thing that I did is pretty much the same in both situations, but the difference in circumstances explain why we think of the scenarios as being morally distinct.
This basic idea is captured by the principle of Formal Equality, which basically says that in order to justify treating two different scenarios as being ethically different from each other, we must find a morally relevant difference between them. In the above example, our intuition seems to tell us that the stranger's aggressive action in the latter scenario constitutes a morally relevant difference from the first scenario, even though my action was the same. If that's true, then it must be the case that other factors besides my actual action itself can qualify as morally significant. And another way of saying that seems to be that we can evaluate different scenarios as different not only when the actor chose a different course of action in one as opposed to the other, but also when the situation in which the actor acted changed in a morally significant way. And that's basically the same as saying that circumstances can be morally relevant, and our ethical rules do not have to tell us what actions are right or wrong without any reference to circumstances.
And I don't think this should be disconcerting. It just seems like an inherent part of playing the game. Any theory of morality which made no reference to circumstances would seem like it would end up being overly rigid and extremely implausible.
Is that a weakness for ethical philosophy? I guess that depends on what you're looking for out of ethical philosophy. If you want a list of actions that you always must perform, or must never perform, with no normative content smuggled in through definitions, then I'm not sure you're going to find any luck. So yea, it would be a weakness if that's what you wanted. But personally, I'd be really suspicious of any theory that proposed to tell me anything like that. Ethical philosophy doesn't seem like it's meant to command us about what to do; rather, it should help us determine what to consider when deciding how to act, and what kinds of things we should care about. And so far as that's the goal of ethics, I see no problem with making reference to circumstances. In fact, so far as ethics should help us in making decisions, and so far as we face different decisions in different circumstances, I wouldn't want an ethical theory that didn't make reference to circumstances!
So hopefully that's helpful, if perhaps not as analytically rigorous as it might be. I'll get to your other questions later.