Modify how Drupal builds fields on a per form basis
I think your third option is the most direct solution since you are only looking to change one form and that file will only be called when you use that form.

The only alternative I can think of is to use hook_form_FORM_ID_alter and iterate through the form items, putting html into the #prefix and #suffix values; or creating #attribute['class'] or #attribute['style'] values depending on what you are trying to achieve.