Joseph, I have to be brief today because I have other matters to worry about today.
Actually, I disagree with your description of sweatpants. I don’t think they are un-zniusdik, as you describe. You can also add culottes (absolutely no outline at all) to this and pants under a skirt. So, we differ on the actual matter at hand. SJS gave you another possibility. The point is that, if this does not come under the guise of ‘lo silbash’, there is much more latitude for allowing it as the concept of what is considered znius is a more nebulous one.
As far as miniskirts and cars. Can a woman wear scanty clothing in her own home? Depending on how you answer that, you can deduce whatever decision you want to make about a miniskirt in a car,although she is less protected in the car, and it might be more problematic.