You said the word "If".
All is, again and again, pure speculation. Or maybe you have some decent sources to share ? (and not from the DM where you seem to spend days and nights).
This book is trashy as hell and Scoby-doo really sounds the brat is truly is.
As expected nothing good will come out from this book, and certainely not, surprinsingly, the Sussexes image. Oh i guess this pamphlet will have some kind of sucess, especially in the US, where the intended readership is.
In the UK it's safe to say that the Sussexes are done. Oh of course some attacks against the Cambridges will make some headlines, but i guess that nothing serious will damage the image of the future King and Queen, especially after their very commendable role during the pandemic.
Because yes timing is everything , and, rightly or wrongly, the Sussexes appear more and more as isolated and sefl-indulged people who spend ages in legal battles in a World falling apart. On the contray the Cambridges , with the old moto "Keep calm and carry on", are more popular than ever.
So i wish good read to all the Harry and Meghan fans and i have no doubt they will find some bitter aguments to continue their quest. But i'm affraid it will be a somewhat lonely journey, as the Cambridges, the Monarchy and the UK have better things to do nowadays and are well above any form of easy controversy regarding another useless family feud.
To summarize this book : "Who cares ?"