1/ You can do that with the menu like you did. You just need to edit the hikashop options of the menu and change the item box layout option to "image and description" so that the description is displayed on the listing.
2/ You can do that with your menu, like you did. Just turn on the "show description" option of your menu and in the main category, set your text in its description.
3/ Only after they click on "choose options".
4 5 6/ You should be able to change that with CSS. Hard to give a more precise answer without a link to that page.
7/ You will have to edit the file of your item box layout via the menu Display->Views to change it. It will depend on which layout you're selecting in point 1/. For the "image and description" layout, it will be the file "listing_img_desc" of the view "product" that you will have to edit.
8/ In modules, you can set a menu assignment. You could just assign your modules to where you want them to appear instead of having them on all menus.
9/ In the checkout tab of the configuration, you will turn off the "login" option to remove the login area, and change the "registration" option to "simplified registration" to remove the username/password fields. For the address fields, you can change them via the menu Display->Custom fields.