Bloody hell 😆 I’m not going to criticise you for speculating about the private lives of the royals, because millions do, but this is next level weird. At least in William v Harry we have some context and basic facts, even if the emotional nuance is mostly private. With this, you have nothing.
When Diana died she was 36 and had only been divorced a year. She was still huge news; most adults could remember her supposed fairytale story and the dream sold in the press, and the spectacular collapse of this was very fresh in the public mind. Had she lived, by the time William got married she would have been 50 and divorced for 15 years. Would she have still been the same major front page of the newspapers draw? Would public feeling have changed? It’s impossible to know whether the Queen, or rather the wider establishment, would have sanctioned Charles’s remarriage while he had a living wife (particularly to the other woman), but it’s equally impossible to know whether the sympathies of the public would have remained solidly with Diana after 10 or 15 years rather than one. People might have softened towards Camilla as they have now; thought that maybe Charles deserved a chance at happiness now, especially if Diana had remarried at any point. You’re speculating that Diana would hate Kate for “supplanting” her, but who’s to say Diana would still have been in that position anyway?
We also don’t know what the 21st century Royal Family would have looked like if Diana had lived. Her death and the public reaction to it forced modernisation. Would that have happened if she’d remained a disgruntled ex-wife? We don’t know if the establishment would even have supported William marrying a commoner, or whether the monarchy would have doubled down on the dukes’ daughters or foreign princesses route. Diana might never even have had the opportunity to love, hate or feel anything in between for Kate. Or maybe she’d have pushed for William to be allowed to marry for love.
Speculation is one thing, but you’ve made the massive assumptions that a) nothing would have been different if Diana had lived and b) her standing with the public would never have changed.