Buckingham Palace aren't the least bit worried Meghan will say anything nasty about Kate, the disingenuous bastards that the Palace are.
They know perfectly well that Meghan and Harry know of the Rose Hanbury affair, something Harry was reportedly very upset about (as he's known Kate a long time and Diana was treated similarly) and no doubt a feminist would consider Willo public playing away with one of Kate's own set, and her limited options to divorce and keep her small children, to be deeply archaic and unpleasant.
In other words, Harry and Meghan are likely highly sympathetic to Kate's position - a gilded cage.
It was rumoured that Kate was considering divorce. We all remember the "flinch" video, where she jumped from Will's touch. Had she made enquiries, her prospects for a fair settlement and custody of her children would have been made starkly plain to her.
Therefore, Buckingham Palace aren't worried about what Meghan will say of Kate.
BuckPalace are shitting themselves worried about what Harry will disclose about William and his conduct towards Kate.