On adding, I've heard 3 days and I've heard 5 days. I'm not sure what is correct, but I wouldn't let it go until the last minute. I know that you are not supposed to be able to add it on arrival...although I don't know why.
I'd use the same timeframes for cancellation, although I think you could really just say no thanks at checkin. I assume if you decline to pay for it, they would cancel it. But I think I would call Disney Dining and be sure. You know what happens when we assume!