A couple of questions:
1. Why would the Duchess of Sussex need an excuse not to go to Great Britain? If she doesn't want to go, she doesn't want to go.
2. Has the Duchess of Sussex been legally (as in listed as a co-plaintiff) to any of the Duke of Sussex's security suits?
3. Has the Duchess of Sussex ever discussed being afraid of an attack while in Great Britain? I remember her discussing mental anguish due to her position and harassment by the press, but never fears of actually harm coming to her, although I may be wrong.
It seems to me, especially giving Great Britain is his home and their vastly different relationships with the press and the deadly consequences of their extreme attention to them before they even met, it's the Duke that's pushing the security issue, not the Duchess.
The Duchess is probably done with the whole thing and is okay with never stepping foot in Great Britain again. The Duke is the one who both has the deep desire for his family to visit his home country and the deep fear that something might happen to them if they do.