Hi,
a. Guest checkout is already an option that you can activate in the backend of HikaShop.
b. I believe that you can achieve the same as amazon with HikaShop. One step for the registration/checkout and one step with the rest and that's it, two clicks. It's just depends on how you configure your checkout workflow.
We're actually looking at redoing the checkout with a lot more AJAX to make it more user friendly in the next versions of HikaShop.
c. You can pass any parameter in the URLs of your website, regardless of whether it is an affiliate link or a HikaShop page.
For example this link:
www.hikashop.com/forum/2-general-talk-ab...html?source=facebook
It will point to this same page but with an extra parameter. If you have Google Analytics, you should be able to track that extra parameter.
Then, there are probably joomla extensions to track these parameters as well, but I'm not a specialist in that domain so I don't know.
d. HikaShop doesn't have that knowledge, but I believe that this should be done by tracking/analytics Joomla extensions which are specialized in that field.
PS: thakn you for your feedback, from a USA perspective.