I am currently assuming that the textures we choose should be ones that actually have significant cast shadows to draw. Which really just means any textures that can be considered to have additional forms on it that are capable of projecting a shadow onto another surface.

This is correct, but that encompasses all textured surfaces given our definition. Texture itself is the presence of 3D forms that are present on the surface of the object. Small forms can still cast shadows - although I'll elaborate on this a little further down.

Textures like metal and chrome, for example, would be "disqualified" since drawing them with only cast shadows would amount to drawing nothing at all.

Metal can have a texture at a very small scale, but chrome is by definition extremely smooth to the point of reflectivity (which is the absence of texture, as reflections only occur when the surface is so smooth that light rays are reflected back in a consistent direction, rather than dispersed by irregularities along the surface). I did include chrome in the dissections example, but this will be removed when that portion of the course is addressed via our video/demo overhaul, which is progressing very slowly, but is progressing. Due to the way the course has developed - starting as a more general regurgitation of what I learned in the Dynamic Sketching course I took with Peter Han (in which texture had nothing to do with cast shadows, but was more a matter of learning how to create different visual patterns with hatching, stippling, and other repeating arrangements of marks), and then being gradually (and fundamentally) redesigned to focus on the very specific and narrow area of spatial reasoning - some of the demos still haven't quite caught up to those changes. This is something we make up for in our feedback, with areas like this being deemed of lesser priority (due to being less about getting you to do the thing correctly, and more about helping you look at different spatial reasoning problems so as to provide a sense of direction for when we focus on developing that understanding in the constructional drawing exercises from lessons 3-7).

In other words, students very much aren't expected to be equipped to tackle texture correctly right now, and that's not a problem in the grand scheme of things. You're just expected to give it your best try, given the tools and information you have right now. It's for this reason that the dissection material is considerably older than the texture analysis material - according to our triage priorities, it's not a huge problem (although admittedly it can be a little demotivating for students).

In a similar vein, the face of a kiwi (the inside when cut in half) would also not be a "good texture" for this exercise because it also has no cast shadows, as its surface is pretty much smooth.

The surface of a kiwi is actually textured. Its surface features pulp which is very slightly bumpy, and channels in which the seeds sit. These irregularities do indeed result in texture.

Now that doesn't mean that there are no cast shadows whatsoever in this image, but they are so ill-defined (at least for my untrained eye) that it doesn't seem like a great fit.

I'm getting the impression here that you're focusing on trying to identify cast shadows in your reference images with the purposes of being able to copy them over strictly by observation. That is a very common mistake (given what I've stated above about the development of the course), but as discussed in these reminders, the cast shadows we're drawing are based on the understanding of the structures we observe as they exist in 3D space, not based on copying them directly.

Now, the explanation I promised earlier, is to do with the diagram you provided with the light source marked out above the texture in profile. You're very close in your interpretation with those diagrams, but there's one issue - the textural gradient we draw for the texture analysis exercise has the light source on the far right. This means that at the furthest right extent of our gradient, everything is solid white - no cast shadows are drawn, because the textural forms themselves either obfuscate them (as shown on the far right of scenario 1) or they are obscured otherwise as in scenario 2 due to the angle at which the rays of light are hitting it.

Pushing further and further to the left, the cast shadows become more and more visible - but having the light source hovering over the texture as you do makes it seem like the cast shadows only start appearing at the very far left side of the gradient.

Instead, if we push that light source further to the right as shown here (this is a diagram I had drawn ages ago, so it doesn't perfectly match the situation you drew, but since even in your case you've got the green cast shadow showing up on the far left, you appear to understand that the shadows do increase the further away from the light source we move), we can see that those shadows do indeed expand the further out we go.

So, even in those surfaces with very slight textures, you can scale it as you require. The important part is that there are shadows being cast, and that these are shadows you can leverage to create a gradient of light to dark, from right to left.

But of course, this is not something students are necessarily equipped to tackle successfully, but rather by assigning it here we're giving you additional context for the spatial relationships we'll be exploring starting from Lesson 3. Same goes for the intersection lines in the form intersection exercise later in Lesson 2 - you're not expected to be able to nail it, but we just want you to start thinking about how forms relate to one another in 3D space. In the case of texture, the shape we design for the cast shadow defines that spatial relationship, bringing everything back to the core goal of developing our spatial reasoning skills throughout the course as a whole.