-Regarding your first question, yes it helps a fair bit to focus your use of line weight on a specific task. Within the context of this course (given our limitations in the tools we're using here), it's generally most effective to focus the use of line weight on clarifying how different forms overlap one another. Cast shadows can definitely help with this, but line weight applied specifically where the overlaps occur (as shown in the reference I put above) is also an extremely useful tool.

-And now regarding th second question, the thing to keep in mind is that texture is not actually all that different from everything else we're doing here. Just like construction, it's about defining the relationships between different forms in 3D space. The only difference is that these forms are so small and numerous (usually) that if we used the same constructional techniques to capture them (explicit markmaking, outlining them in their entirety, drawing them directly, etc.) our drawings would just end up incredibly noisy.

This is what most students miss out on - they think, "well I'm just going to draw the shadows I see from my reference image", but this is incorrect. Drawing what you see in your reference does not mean you understand what is depicted there. It doesn't mean you've noted the presence of each little bump, of specific scratches, of specific cracks, etc. It simply means that you're mindlessly copying 2D information over from the 2D reference to your 2D drawing.

Instead, we strive to use the visual information in that reference (cast shadows among other things) to understand the nature of those forms, and then using that understanding we create our own shadows. Much of the time those shadows will match what's in the reference, but that's not really important. What matters is that those shadows define specific relationships. That their shapes are designed with purpose and intent, because you know what specific forms and surfaces are being linked through the shadow's creation.

Again - this is not easy, and the very fact that we can't outline the textural forms first before putting down our cast shadows makes it incredibly difficult. All the same, don't avoid it because it's hard. It will get easier to wrap your head aroud as you continue to build up your spatial skills via constructional exercises, but you'll develop it faster by attacking the problem from many angles. You have shown already that your observational skills are quite developed so at this point it is not a matter of skill, but it's more about how you approach it. I hope this clarifies your doubts, keep up the good work