Yes, first I try to turn on auto verify and use {captcha} and it only show the captcha, not act on it.
Then I tried the code <?php global $mainframe; $mainframe->triggerEvent('onCaptchaDisplay'); ?> with the same result. I can see that you have write 'onCaptchaConfirm' while I have wrote 'onCaptchaDisplay'. I will try to see if it helps to change the word.
PS. How do You made the captcha verification on your Forum page?