After giving it some thought, I realize the Not Sure option shouldn't be much of a problem. We do have some members who may abuse it, but we'll have to be clear that a Not Sure response will be treated as a Not Attending and we will be making accommodations for Not Sure responses. We are a homeschool group and in general, I have noticed that homeschoolers are not great at showing up to things. We can at least now boot those who make a habit of no-showing to events.
As for the blank email, I sure hope that can be fixed otherwise it will cause all sorts of confusion with my members, and I'll be fielding questions everytime someone gets one fo these emails.
Thanks for all your thorough testing!