Can't attach #ajax to altered form displayed in block
drupal_get_form will return a builded form/ rendered form. i.e Your form has been already prepared based on the specification done in the form callback, like its finalized.

So making changes to the return values of drupal_get_form wont work.

If you want to alter any of the form then you should use any of the following hook


hook_form_alter
hook_form_FORM_ID_alter