The thing is, this is to some extent browser specific (all browsers render slightly differently) and also depends on browser preference settings, so you can never guarantee that the nav is not going to go to two lines.
Secondly, Pilcrow is what is called a reactive or responsive design. If you grab and resize the width of your browser window you will notice that the theme narrows with your browser window and it has a maximum width setting to keep it from getting too wide. That means that the width of someone's browser window, or the pixel width of the screen they are viewing it on affects whether the menu will be one line or two.
The above is sort of the long way of saying that there really isn't a solid fix for this behavior.
That said, you can adjust the letter spacing for the menu by adding this and then adjusting the 0.3em letter spacing value.
#nav a {
letter-spacing: 0.3em;
}