There are so few occasions for wearing tiaras that it's difficult to avoid debuting new pieces at state occasions. The addition of the RVO for Catherine will attract lots of cover anyway so she could add Diana's sapphire choker without it being seen as drawing attention from the Queen.
If Meghan attends, it almost doesn't matter what she wears because as the most recent HRH, there will be lots of coverage of her anyway so again, she could debut some of Diana's pieces without increasing the attention. I doubt she'd be loaned any tiara other than the Queen Mary's bandeau she wore at her wedding.