It doesn't happen too often but this is one of those times that I really, really wish I could play the video and hear it. I'm going to take all your words for it that she did a marvelous performance.
The more we get to know of of Catherine and get to know her more and more over the years along with William, the more I realize that this is a couple that has their heads screwed on the right way, have their priorities in order, have the "right stuff" for what it takes to represent people to be leaders and eventually monarch and consort. They both also have that inherent trait that Granny has which has made them realize that blowing their own horn as far as their abilities and talents tends to give you an empty audience.