Not sure why your widgets aren't showing up. As Juggledad mentioned, the widget area is there, but there are no widgets in it. I take it you added widgets? You will have to add the widgets while Montezuma is active, I don't think the widgets you added through Atahualpa are going to show up.
As far as the width of the single posts go, what is causing the problem is a plugin that's being used to display a Google AdSense ad at the bottom of the post (right after the social media buttons). That's why it's not affecting the home page or any category/archive page, because it's configured to only display on single posts. The ad has a fixed width of 728px, so it's extending the width of page. The ad itself is not responsive, so it doesn't shrink in proportion to the screen width. If you disable the plugin, you will see your mobile pages shrink correctly in width. Or, you can try adding this rule to the end of your various.css file:
Code:
.wpInsertInPostAd {
overflow: hidden;
}
This will hide any part of the ad that doesn't fit in the page, if that is acceptable to you. Otherwise, you can use a media query to hide the ad altogether on mobile screen sizes.