I definitely think Kate has had elecution lessons. She is tense and self conscious in interviews too. Zara does not speak like that , she sounds pretty normal. It's not a given that anyone who goes to a private school sounds 'marbles in the mouth'. My OH has three Aunts who all went to Cheltenham Ladies College, and they literally do speak like they have marbles in their mouths. It was a thing back in the day, but not so common now. I think Kate still thinks it's what you do.
I thought Kate looked lovely today. How HM still keeps going is beyond me. How many other women in their nineties are effectively still working?