Yitay,
Your description of tznius as a general concept is very nice, and I assume it is correct. However, that does not mean that everything that in modern times we refer to as “tznius” fall under that same rubric.
For example, the requirement for a married woman to cover a portion of her hair, is d’oraisa, applies regardless of norms of the time, and has nothing to do with hirhur.
Similarly, the requirement to cover “osso makom” is not because of hirhur (it applies to ones spouse as well). The Bais Shmuel on the Rama in E”H 25 says that the Rama did not mean to allow the “something else” that you refer to and therefore it is not ???? ????? ???. You will probably argue that the Bais Shmuel is dochek and it is. But that is the accepted view point of the majority of poskim (e.g. the Aruch Hashulchan, Chachmas Adam etc.) and therefore is the ikkar hadin.
You are taking your assumption of that all tznius regarding covering up body parts is based on hirhur and you are putting into the Gemara. Without a source for that assumption, you are presenting a chiddush as well. The fact that it is clear to you does not make it the pashtus. There are certainly things that must be covered up because of hirhur but there are also things that must be covered despite no chashash hirhur or where hirhur is mutar.
I can’t say for certain why Chazal would make “shok” like “osso mokom” but I can speculate. There is an aspect of tznius that is based on human dignity. A person should be dignified and embarrassed by certain displays of baseness. This is how many of the poskim explain ???? ?????? ????? ????. Chazal said that with respect to arayos (and even one’s spouse when davening) we will treat the “shok” like osso makom, as a place for which it is debased to reveal and untzniusdik to look at, even when there is no daas and no chashash of hirhur.
Why didn’t Chazal make this rule apply to one’s spouse as well? I can’t say for certain, but perhaps because it would interfere to greatly with the mitzvos between a husband and wife.