> they click the link in the email that comes after the opt-in questionnaire
You could put some part of the first questionnaire wave into a questionnaire that comes after that click. When doing so, you'll need a dummy mailing that is never sent, but allows to make the first real mailing a follow-up that, again, can respond to the response status.
Or, you could simple put the mailSchedule() command on the last page of the first questionnaire. That may be an even more simple solution.