These all look good to me. I think you have the right ideas. I personally also struggle a lot to draw folds as Marc intended and lean to simplify and reconstruct with an eye on the references more so than copying the shadows.
Either way it helps to understand the folds.
The only piece of advice I could share was that while doing these exercices, at one point I tried to construct clothes completly from "imagination", and just follow the points of tension rules, foregoing references to dress the naked people. It didn't look that good.
I switched back to look at references if only to "construct" new clothes by looking for clothes in similar poses, or even posing them myself and taking a photo, and the result was almost night and day.
So the lesson I learned then was to just keep using references until I gain more experience, even when "constructing" new clothes.