Let us see if we can work out a compromise here. We must agree that there is nothing wrong with any garment that covers the required halachic areas. There may be other reasons why a person would or should wear better clothes. For instance, I never wear blue jeans or shorts , since I don’t think it is bkovodig. However, there is certainly no halachic reason why blue jeans should not be worn.
Instead of calling these issues tznius, which they are not, maybe they should go under another name. I have no clue what cap sleeves are or what slingback shoes are. Nevertheless, if a person feels demeaning when wearing them, then by all means don’t. If you want a higher level of professionalism, then wear better clothes. However, do not look down at people who follow halacha. Do not look down at people who do not follow halacha either. I have very good friends who do wear jeans and shorts. To each his own.
If a woman wants to wear short sleeves over long sleeves, then why not? If she feels it is too casual, or makes her feel silly, and she has a responsible position, or even just for her personal tastes, then don’t. However, let us not drag tznius into this, as it is strictly a matter of taste.