Unable to load css files using ['#attached'] or drupal_add_css
The pathing was wrong after all. The preceding "/" was messing things up. Strangely, this was not an issue for javascript attachments. For example:

$form['#attached']['css']['css2'] = '/sites/all/libraries/data_search/tagme/tagme_jquery-ui.css'; //fails
$form['#attached']['js']['js2'] = '/sites/all/libraries/data_search/tagme/tagme_jquery-ui.min.js'; //works
$form['#attached']['css']['css2'] = 'sites/all/libraries/data_search/tagme/tagme_jquery-ui.css'; //works