Artk, I especially loved this part of your post:
If we take it to its extreme, though, it makes etiquette another tool for bullies and boors to run the show. A "polite spine" is just that -- polite.
I have a tiny possible (probably not) disagreement, though -- I'd agree that "How kind of you to take an interest" is an etiquette-sanctioned response to an inappropriate question. But I wouldn't say that it is therefore always
"entirely polite." I can think of circumstances in which it might not be. Maybe the question was innocent or just ignorant or pretty minor, or maybe the tone was ultra-frosty and accompanied by a withering glare designed for maximum humiliation of the questioner, loud enough to be heard by as many people as possible. I think that would be a rude use of etiquette, especially with regard to innocent bystanders who could be embarrassed or a host whose party you'd be bringing down with drama. I prefer to change the subject -- believe the ehell term is "beandip" -- instead if possible. Other times I ask a variation of "Why do you want to know?" in which I try to make it sound like "If I know the reason for your question, I can give you a more useful answer," even though I'm really just trying to avoid the nosy question.
Anything that is designed to teach a rude person a lesson by giving a snappy answer or humiliating them usually backfires, in my experience. Instead of examining their own behavior, which they might have done otherwise, they feel defensive instead. No one learns much when they feel stupid and called out.
Take the Margo & Claudia string, which is likely the one you meant. Of course Claudia was wrong. But I imagine her takeaway from the evening was to think, "I can't believe Margo treated me like that! She said it was okay for the boys and me to stay, but all evening she made it clear she resented it. Why didn't she just say "sorry, no" in the first place? She knew I would've been okay with that. I guess I should've called ahead, but I didn't deserve this kind of punishment for staying when she told me to." If Margo had either privately explained that she wouldn't be able to accommodate Claudia and the boys in the first place, or else privately discussed the food situation and possible solutions with Claudia and asking Claudia to help her reset the table and fix tuna sandwiches before the other guests came in to the dining room, Claudia would have seen how her rudeness created problems for Margo as a host without getting distracted by her own hurt feelings. She may be unteachable. But if not, I think there would be a much better chance of educating her that way than by behaving in a way that is correct per etiquette but nevertheless PA and shaming (albeit very understandably!).
So my question is, does good manners, if not specific etiquette rules, require us to take the path of least harm when we have a choice? Some etiquette rules seem to suggest that -- such as replying "I'm sorry, I have other plans that day" instead of "No, I don't want to come to your party because it will be boring" or not mentioning in a thank you note that you received 5 items identical to the person's gift.
Does provocation or others' rudeness change anything? If I'm going to say "How kind of you to take an interest," should I say it as nicely as possible or is it equally polite to say it with a poison glare? If someone directs a sexist or bigoted comment at me, is it just as polite for me to scream at them that their hate disgusts me as to simply excuse myself, blink in stunned silence, or just say quietly, "Wow." Does it matter whether the question was "Wow, Gellchom, you sure do sneeze a lot; are you sick?" or "Wow, Gellchom, you sure do eat a lot; is that why you're so fat?"
To me, your "a 'polite spine' is just that -- polite" says it. You stand up for yourself, but you do so with your best behavior, which may be more than simply following an etiquette rule.