unable to force JS library to HEAD
I think you've indented too far. The line following header: true is directly beneath it in the example you linked to.

global-styling:
  version: 1.3.2
  css:
    theme:
      includes/mapquest/css/mapquest.css: {}
js-header:
  header: true
  js:
    includes/mapquest/js/mapquest.js: {}