Creating a large full width "recent post" section

I would like to try to get my layout to have 3 top menus (1 page menu above header, 1 page menu below, and 1 category menu below the page menu) and have the menu border only have one side.

The main part of the layout that I would like to do is to have my most recent post be larger than the rest with the excerpt (not full post) and have it stretch across the width of the page and over the sidebar.

I have been searching for ways to do both of these things and have spent hours plugging in html/css in the section to insert html/css as well as trying to make changes to settings. So far, nothing has worked including several things I thought would work and others had success with.