1. The HTML that defines the page structure are in the *_template blocks, in the Blocks Admin Tool. In the default install, the main part of the page is in a 99%-width table, while the header is in a 100%-width table. This doesn't go edge to edge because as I recall, in most browsers the body element is rendered by default with a small margin (or maybe it was padding) to keep text on the page from running into the edges of the screen.
2. You want to look in the Sections Admin Tool. Load up a section, then tweak the section permissions for each section (at the bottom of the page) to give your admins "Auto-post to section" and your regular users "Deny" for story posting. Diaries by default don't show up on the front page. Permission to post stories and diaries are given by the same permission in the Groups Admin Tool (story_post) so you have to set "Deny" on each non-diary section if you still want your users to be able to post diaries. It would probably be a good idea to edit the user_box (in the Boxes Admin Tool) to take away the submit story menu item, because it only checks the story_post permission. This answer can be found in the Scoop Admin Guide  in the "Features" section, as "Flexible Sections and Categorization"

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Discuss the art and craft of writing