“That aside, in the time to which you are referring, there was not a widespread frum female educational system like there is today. The bar is higher now. If virtually all frum girls are torah-educated and versed in chumash, Rashi, Rambam, and other varied Jewish subject matter, including halachos, why aren’t they all dressing within normative tznius guidelines – even within the spirit of halachah?”
So you think the halachos changed? That’s the problem – people turn chumros into halachos, and expect everyone to follow them. If you don’t, you’re looked at as a sheigetz/shiksa. Keep to the basic halachah, and don’t put unnecessary chumros on people. It only leads to people resenting them, and not following even the basics.