Basically the answer to your arrow question is that you want to be thinking in terms of 3D space and not 2D. The easiest comparison I can bring up is our boxes, if we constructed an actual box we would want it's pairs of edges to be parallel to one another and of equal length or else we'd have a misshapen box, but once we view that box from an angle or draw it it will converge to a natural vanishing point due to perspective or the vanishing point we create on a page.

So when drawing an arrow we want to be drawing the same curve in terms of it's position in 3D space, however because it's moving towards the viewer it foreshortens and widens consistently as it gets closer and the edges diverge. Remember that while this occurs the space between curves of the arrow will widen as well.

Textures are difficult and we don't expect you to do them well here, it's mostly just an introduction to the concept and ever texture is it's own challenge so don't stress about them too much, they just require a ton of mileage. One thing to remember is that if we point out a flaw it doesn't necessarily mean every single one of your attempts has that flaw, your scale analysis and corn dissection as quick examples are definitely steps in the right direction. While if we look at dissections like your turtle skin, or grass there is more of a focus on outlines, and if we look at your pavement or meat attempts there's more focus on negative space and colour changes (and even form shading in your strawberry attempt).

Overall your submission was solid and it's great that you asked questions and want clarification on things, I just hope you're not over-thinking or stressing yourself out too much.