How about removing some clicking steps, why not offer the reply links (optionally) straight in the email? Like:
=== Click here if you are attending http://inviteup.com/i/abc123/YES
Click here if you are NOT attending http://inviteup.com/i/abc123/HELLNO
Click here if you MIGHT attend http://inviteup.com/i/abc123/PERHAPS ===
Also, I agree that you are on the right side of complexity. Start clean and add what is lacking.