Font-Face changing via JavaScript

You can create a new <style> element with the @font-face rule and append it to the document's head:

var newStyle = document.createElement('style');
newStyle.appendChild(document.createTextNode("\
@font-face {\
    font-family: " + yourFontName + ";\
    src: url('" + yourFontURL + "') format('yourFontFormat');\
}\
"));

document.head.appendChild(newStyle);

Of course, you'll probably need to provide all the necessary font formats and URLs, too, unless you're only worried about support for modern desktop browsers (in which case you would just use WOFF – I assume that's reasonable, because of the other features you mentioned).

Define a FontFace object:

new_font = new FontFace('conthrax', 'url(fonts/conthrax-sb.ttf)')

Call its load method to download the font:

new_font.load().then(function(loaded_face) {
    // use font here

}).catch(function(error) {

});

... this returns a Promise, which when resolved passes the loaded FontFace.

Add the loaded font to the document:

new_font.load().then(function(loaded_face) {
    // use font here
    document.fonts.add(loaded_face)
}).catch(function(error) {

});