benignuman –
It is not uncommon for halachos to be derived from diyukim in Shas.
If we would derive an particular case of borer from a psak an amora made about nosein ta’am lifgam, I could hear. But if we would derive the entire concept of borer from that secondary place, I would find it extremely strange. Same here. You don’t hear a word about specific guidelines for dressing; it’s kind of hard to believe they just forgot to mention it.
That such minhagim are binding is clear from the Gemara in Pesachim I cited
Find me a source that someone has to follow a minhag neither they, nor their parents, nor everyone their community ever followed.
where I live the frum women still cover their arms and certainly their bodies and legs.
In many communities it is normal for observant Jewish woman to adhere to a very different standard than that of certain parts of the NYC metropolitan area. Whether it is skirts that just barely reach the knees, sleeves that don’t go all the way to below the elbow, or completely sleeveless, there are many communities where such is the norm among the (relatively) observant Jewish population.
As for your final point, about Rashi:
While the Pri Megadim and the Mishna Brura are certainly not to be taken lightly, and you are entitled to hang your hat on them, you are suggesting something which is far from the pashtus.
First of all, what is the svara that ???? ????? should be assur? If the svara is that things that are normally covered will lead one to hirhur by merely a glance – then obviously it is subject to the time and place, and not just to the Jewish minhagim either. Whatever people are used to seeing is not ervah, and whatever they aren’t, is. In fact, the Pri Megadim himself acknowledges that there’s no such thing as an “absolute” ervah: ???? ????? ??? ????? ???? ????? ??? ???? ????? ???? ??????, ?? ??? ???? ???? ???? ???, ???? ???? ??? ???? ??? ??? ?????. Therefore the whole ervah Gemara should bichlal not apply to a place and time where exposed hair and legs are prevalent! And if you want to say otherwise, that there is such a thing as a shok being an ervah even where people see hundreds of them every day wherever they go, why??? It’s a ???? ?????.
Second, if Rashi actually meant that, then why did the Gemara need to come on to krias shma? This understanding puts words into Rashi’s mouth which force the Gemara away from the pashtus.