WYSIWYG module is not finding my custom plugin
Turns out the tutorials and answers were wrong (at least for my set up/needs) The extra hooks and $settings['extraPlugins'] = 'button_styles'; were probably causing conflicts. I removed all but this;

function MY_MODULE_wysiwyg_plugin($editor, $version) {
switch ($editor) {
  case 'ckeditor':
    $module_path = drupal_get_path('module', 'MY_MODULE');
    return array('my_plugin' =&gt; array(
      'path' =&gt; $module_path . '/plugins/MY_PLUGIN/',
      'buttons' =&gt; array(
        'style_1' =&gt; t('Style 1'),
      ),
      'load' =&gt; TRUE,
    ),);
  break;
 }
}


and now it recognizes my plugin.js.