The important point you may be missing here is that our goal here is not to reproduce the reference image at any cost. As I mentioned in my initial critique, each of these drawings is just an exercise - an opportunity to have our brain attempt to think in three dimensional space, even as the marks we put down exist on the flat surface of a two dimensional page.

So, when you ask "What is the correct way of doing here? If I absolutely had to subtractively construct this part, should I make..." the issue is that within the context of this course, approaching these constructions additively is part of the exercise.

There are absolutely ways to work subtractively while continuing to work in 3D space (as shown in this diagram - you'd take your existing 3D form and split it into separate parts by defining the separation with a contour line that wraps around the 3D structure), but again - for the purposes of this course, it's not really important right now.

In Lessons 4 and 5, we're just getting used to the idea of all the forms we add to our constructions actually existing strictly in 3D space, and we avoid jumping back and forth between 2D and 3D because of the goals of this course. In Peter Han's Dynamic Sketching - the course this one was built upon - he definitely jumps back and forth between forms and shapes freely, and achieves a lot of really fantastic results, but when I was taking that course many years ago, I definitely noticed that some students (those who had stronger spatial reasoning skills) managed to apply what Peter was teaching just fine, and others (those who didn't have as much experience manipulating 3D forms in their heads) basically just fell flat.

So as I developed Drawabox over the years, I shifted more towards focusing completely on building spatial reasoning skills, and thus I push students more towards working in this manner. Always remember - it's never about specific cases like the ones you listed in your response (which is why I'm responding more generally, rather than addressing each one). It's about the underlying purpose of what we're doing, and how it's meant to develop your skills well beyond the resulting drawing. The drawing itself, once finished, doesn't actually matter.

One thing I should mention though is that if something's actually already flat - like a leaf, a petal, or an insect's wing - then modifying its silhouette is fine. It's just when those forms actually have volume to them (which is the majority of what we deal with here) that we want to ensure the volume is maintained throughout, so we can continue to understand what we're working with as it exists in three dimensions.

I get that this can definitely be confusing, but keep pushing forward. It's entirely normal to see these issues in Lesson 4 (and to be fair, these "rules" are things I haven't fully incorporated into the lesson material proper yet, and am currently only sharing in critique - it'll be added when my overhaul of the course material/videos reaches this far). I still mark the lesson as complete because you will have ample opportunities in the next one to try to apply these principles. If you continue running into issues there, we'll deal with them then.