What possible threat could this woman be?
She'd already been very effectively neutralized. When it comes to women, it's easy. Paint them as at best pathological liars, exhibitionists or neurotics and at worst paranoid or mad. You then remove their voice as not a word they say is ever taken seriously.
It's the age-old way of silencing women who have become superfluous or a liability, or simply unwanted, as evidenced by the many women in history whose husbands had them shut away in mental asylums. It's recently come to light that Charles Dickens tried to do this to his long-suffering wife - which given the way he allegedly treated her doesn't exactly surprise me.
Diana was a case in point. So, more recently, was Charlene of Monaco. Royalty PR is frighteningly effective - take them on and you lose, as others have found out before Diana and after. Of course, not one single, solitary word that comes out of the mouth of Diana's younger son's spouse could possibly be the truth, could it? Windsor-PR and their media sidekicks have done far too great a job at discrediting her - so much so that it seems to occur to few people that she might have had even one legitimate complaint. Simple common sense should dictate that life is rarely that simple or one-sided.
If anything's a conspiracy 'theory' surrounding Diana, it's this. Why go to that trouble and then bother to kill her? No one gains. Had she lived, she'd doubtless simply have become a bit of an embarrassment like Fergie. And it isn't strictly a conspiracy - merely a symptom of a tediously, predictably misogynist society which is far more happy to believe women are crazy fantasists than that they might - shock horror - be victims of abuse, smear campaigns or the usual systemic class and gender inequalities.
Plus sa change ...