The feature content is in the feature.php file. It's already set up with the feat_box divs, which are essentially the boxes that hold the content. You can use any HTML in inline CSS to create your feature content. I tried to style the default content a little bit just so you could see how to use inline CSS.
Yes, activating a widget will replace all the default content. You can activate several text widgets and insert HTML code if you want. It's basically the same as editing the feature.php file.
Same thing with the footer, just edit the footer.php file or activate widgets.