It is not a border but the drop shadow of the wrapper that ends up on top.
Remove your border:none rule in the post footer BG CSS+ box and instead use the rule below in that CSS+ box
{position:relative;z-index:10;}
Xtreme has One footer area, but you can have as many widgets in it arranged any way you want, so you actually an infinite number of possibilities instead of just 4.
You can use a number of columns, so if you want 4 just drop 4 widgets and use column:4 in the settings. But if you have plus, you can actually create any kind of layout with rows of widgets with various width, and even control ho the layout changes on small tablets and phone. Have a look at the demo site footer widget area and how it changes when you reduce the browser size
http://demo.weavertheme.com/insertion-areas/
and read the tutorial on how to create these
http://forum.weavertheme.com/discussion/10342/weaver-xtreme-tips-and-tutorials-part-1#Item_1