Can my responsive Nav be made simpler?
Multiple nav elements

While multiple nav elements are allowed for any given section, it looks dirty to me.  Consider using a single nav instead.  If you need an extra element as a style hook, then use div or section.

Duplicate content

Having content that is only visible to mobile users and similar content that's only visible for desktop users is code smell.  This one is extra bad because it uses identical markup for both.  Just use one set of markup and have CSS style it differently depending on the viewport.

Using px for the width of text elements

Text is not something you as a designer have any control over, all you can do is make suggestions.  An element that's 210px wide might look good for text that's 16px Times, but what happens when the user has specified that they need 20px Segoe because their eyesight isn't as good as it used to be?  Using em/rem is a better choice in most situations.

Using px for media queries

When you use px for your media queries, they'll fall apart when you start zooming (not sure if this is still the case).  Using ems are more reliable, especially if you're following my last point.

See also: http://bradfrost.com/blog/post/7-habits-of-highly-effective-media-queries/

Redundancy

When you float an element, most display values are ignored, including inline.

.main-nav ul li {
    display: inline;
    padding-left: 28px;
    float: right;
}


On a relatively positioned element, a top/right/bottom/left value of 0 does nothing.  I don't see anywhere where this is overriding a previous setting either.

.menu {
    overflow-x: hidden;
    position: relative;
    right: 0;
}