multiple submit handlers
All you need is $form['#submit'][] = 'custom_redirect';, if you want your custom submit handler to be ADDED to the default submit handlers.it will run along with the other (user module submit handlers) this way.

If you need this function only as your submit handler, then 

$form['#submit'] = 'custom_redirect'; (assignment not addition to the submit array)

No need to alter the $form action.

Just alter the original submit button title if its necessary, no need to add another.