I think it is possible to see this as one of the ageless hells - having a child who behaves monstrously but is impossible to control through the usual leverage.
Andrew was evidently her blind spot. But also it is hard to know what might have worked in controlling him.
It looks to me like Andrew has some kind of genuine psychological disorder compounded by an unforgivable indulgence by his caregivers in his formative years. This might explain his extreme behaviour.
He was also sexually abused as a young g boy according to Andrew Lownie’s book ‘Entitled’ ie taken to an older woman when still a young boy. Something of an aristocratic tradition but none the less potentially hugely damaging for a child.
Her massive mistake was giving him the trade envoy role - presumably as a tragic hope that he would rise to the occasion and become a responsible adult. She must have been doggedly turning a blind eye - she was certainly informed about his behaviour but every lackey was also trying to hang on to their cushy royal job.
She clearly loved him but was also responsible for how he turned out. What could she do given that he probabl lied about everything. She could have him drugged and locked up? Honestly it does seem like the only way such a sexually and financially compulsive and amoral character might be constrained.
Nothing worked short of what they have finally done, too late of course.
I think the wider family have always known but his behaviour is like one of those really badly behaved dogs who knock you over, hurt you, crap on your property, bite you etc and the owner loves them too much to see how badly they are behaving - if they did they would have to act decisively and effectively.
Rob Reiner & his wife were frightened by their son but maybe loved him too much to act effectively. Maybe they kept trying and failing.
AMW is also symbolic of all the privilege and entitlement of the family.
I think that they need to change quickly to survive. They could for start :
Open your wills.
Stop destroying the historical record.
Be transparent.
Establish a royal register of interests.
Pay real tax.
Pay death duties.
Open your finances to parliamentary scrutiny particularly the duchies of Cornwall and Lancaster.
Stop the grace and favour deals for non working royals.
Clarify the terms of the Crown Estate and make them favour the nation not a family.
Make it clear that the law applies to them too.
Do the right thing now.