Oh yeah there's a much better solution.
On the first trigger that asks for the Y/N reply, do this:
a. add to group "no reply"
b. send the initial question
c. send the delayed follow-up "we've not heard from you" which is personalised to only those in the "no reply" group
When someone replies to the question, remove from the "no reply" group
Now when the delay expires, only those who haven't replied get the message.
About the 30 second delay... it might not be long enough because of the way the cellphone network works. Conducttr will always send the messages 30 seconds apart but the cellphone network has inherent delays and you may find on rare occasions that they sometimes arrive together or less than 30 seconds apart