NY Mom- Oz Vehadar Levusha allows for shells, and that book is known to be very machmir; also, there are some very legitimate reasons for it other than ‘just wanting to kasher the sleeveless’. For example, I didn’t always keep the rules of tznius, and when I started, I was faced with the problem of vetting my entire wardrobe. Nothing could be done about the skirts, and I spent a lot of money replacing my minis with proper, tznius skirts; to do the same with all my tops would have made the whole venture prohibitively expensive- and thanks to a few colored $10 shells, it was totally unnecessary. And don’t even get me started on how frum clothing stores are the world’s biggest rip-off. I still find myself buying sleeveless dresses from regular retailers and wearing shells underneath because for some reason that is beyond my comprehension, heimishe stores see fit to charge me $100 for what is, essentially, the sleeves.
I agree with you that there is a problem with people taking shortcuts with tznius that they wouldn’t dare take with, say, kashrus. I see plenty of skirts in shul that don’t quite make it to the knee, skintight dresses, and custom sheitals that look so natural as to make these women look single.
Custom sheitals are my big pet peeve; in addition to just totally ignoring the spirit of the law, many actually think they are keeping it better (at least when it comes to other issues, like tightness, most will acknowledge that they let fashion eclipse halacha). I wear only snoods and tichels, and someone with a fancy sheital once condescendingly told me that sheitals are really preferable because they cover all the hair while other head coverings do not. I asked her if she could locate a single hair protruding from my snood, which she could not.