If you haven't mailed the letter yet, you might try using some excuse for not making it before Saturday. Like the dog broke its leg, or you were traveling, or if was St.Patrick's Day & you thought the post offices were closed. ANYTHING to make them have to explain their policy. It might just be easier for them to grant your rescission.
I thought maybe I was alone with this thought. Fact: the rescission date was missed. Fact: The certified USPS mailing will PROVE the rescission date was missed. If Grandview refuses the rescission, they can just reference your certified mailing.
I would mail it by regular mail with communication dated during the appropriate time period. Four or five days later, I would send it by certified USPS and reference the earlier MAILED correspondence noting that you are following up as your 'counsel' has recommended.
At the very least, it will give them pause and maybe a prudent employee will suggest they "just let this one go."