To answer your first question - yes, you are welcome to submit previous lessons for official critique in the future. And for the second question, keep in mind that this course has its own specific goals and priorities, focusing primarily on every drawing being an exercise to help develop the student's underlying grasp of 3D space, and rewiring their brain so that they understand the things they draw in those terms, rather than as though it all exists on a flat page. To do that, we rely on a lot of explicitly three dimensional forms here.

When you draw with flat shapes outside of the course however, that doesn't mean that the understanding and rewiring goes away. Rather, it gives you the ability to jump back and forth between looking at what you're producing as being flat, and as being three dimensional, when it suits you. It's true that it is easier to understand proportion in terms of two dimensional shapes (especially when focusing on the negative shapes in what we're drawing), and so that will certainly be an approach you'll be able to use outside of this course to help improve your proportions.

Back when I learned figure drawing from a course with Kevin Chen at Concept Design Academy, his approach was to first start off with a 2D mannequin to establish all of the proportions, then to take those shapes and build 3D forms off of them. So there's no harm in jumping around between them, as long as your understanding of 3D space is strong. In order to build that up, here we're setting aside the flat shapes and working strictly in 3D.

Anyway, moving onto your cylinder challenge, you've largely done a pretty good job. I did notice that as you started the first section - the cylinders around arbitrary minor axes - you were definitely not as confident as you could have been, and this resulted in you making minor mistakes like not drawing through your ellipses two full times before lifting your pen. As you progressed through the set however, this did improve, though there is still room for growth there. Make sure you're applying the ghosting method when drawing your ellipses, so you can invest all of your time into the planning and preparation phases, and ultimately execute without hesitation.

I'm glad to see a good variety of rates of foreshortening here, and from what I can see, you are demonstrating a strong intuitive understanding of the relationship between the two major manifestations of foreshortening. Foreshortening is communicated to the viewer through the shift in scale from one ellipse to the other (closer one is bigger, farther one is smaller), and in the shift in degree (closer one is narrower, farther one is wider). What some students forget however is that since these two shifts communicate the same thing - how foreshortened the cylinder is, and therefore how much it's tilting away from the viewer - they need to operate in tandem, ensuring that one such shift in not extreme while the other is more minimal. Looking at your work, you do appear to keep this relationship consistent.

I'm also pleased to see that you're checking your minor axis alignments consistently, and that you're improving upon the analysis at every step.

Continuing onto your cylinders in boxes, your work here is similarly well done. You've followed the instructions well. This one is actually all about developing students' instincts when it comes to building boxes that feature two opposite faces which are proportionally square. We do this by taking the line extensions from the box challenge, which helped develop ones instincts for making sets of lines that converge consistently, and add to it the three lines from each ellipse: the minor axis and the two contact point lines.

In checking whether these additional lines converge towards the box's own vanishing point, we can see how far off we are from the ellipses representing circles in 3D space, and therefore how far off we were from having the planes that enclose them represent squares in 3D space.

You've done a great ob of being thorough with this. My only suggestion to push this further is to ensure that both minor axis lines are extended fully as far as the others, so you can compare those convergences more easily. In some cases, like box 167 for instance, the minor axis of the ellipse closer to the viewer wasn't extended far enough to make that comparison in a useful manner.

Anyway, you've done well here. I'll go ahead and mark this challenge as complete.