Amazing piece of writing Kim. Thanks.
As a heavy Amazon user myself, I often asked myself how a company with an ugly cluttered UI is so successful? Well, I’ve realized they do a few things right. Looks like the system has great usability for review-leaving users, they have one-click ordering for frictionless experiences and although they have millions of products and dozens of categories, their nested category navigation has a rapid reaction of the dropdown menu, which gives users an instant overview of each subcategory with zero delays.

That said, I think that the designs you’ve created for mobile are much clearer. Also, you’ve created a great tutorial into how one should validate a feature, from finding the user’s pain points up to validating your designed solution vectors.