Programmatically added comment form on node page doesn't work
I'm not sure about the failure to submit, but drupal_get_form caches the form config, so making changes to the #redirect after calling it will be lost by the time the form submits. You'd need to use hook_form_alter for that change to persist.

As an alternative, you might consider the AJAX comments module. In theory that will work with a manually embedded comment form, and it'll sidestep your current issue while providing a slightly better UX for comments site-wide.