Joe – Ask your LOR if they are pritzus or Da’as Yehudis in your community. Those are the reasons why it may be Assur.
LUL – I wouldn’t expect the Mishna Berurah to explain the SA any other way than his own shittah. Others disagree.
Your first source holds like the Mishna Berurah. Nothing new there, others argue.
Your second source specifically says “b’makom shetzarich lihiyos mechuseh”, i.e. Da’as Yehudis. I’ve already proved from the Teshuva in Even Haezer earlier that Rav Moshe holds Da’as Yehudis depends on the place.
Also, as pointed out earlier, no one disagrees that hair needs to be covered in some fashion (Da’as Moshe), and not doing so is Mevatel the Drasha from the Torah (a “sin”, although not a Lav or a Bitul Aseh, l’cheorah).
Nothing new there either.
Finally, Rav Moshe (in that very teshuva!!) says that Ervah for Kriyas Shema has nothing to do whatsoever with what a woman may or may not walk on the street. If you read the teshuva inside (again?), you will see it.