Returning the form in form_alter function
You can use it, sure. But it won't do anything. 

The $form is passed by reference, precisely so that it can be altered in-place. Core never does anything with the return value from form alters, so there isn't much point doing it.

Your code looks right, put a drupal_set_message/dpm in the submit handler to confirm whether it's actually being called or not, or whether it's your one-liner that's not doing what you're expecting.