I have been thinking a lot about this lately. And I think it was one of TRF biggest secrets how mentally unstable the Dimwit has been, and how Diana was. I believe it was diagnosed and "under control" with Aitch before he met the ILBW. I believe this is why Wiliam and Catherine were so keen on Heads Up and involving him. Most of their speeches in the beginning were around the stigma behind mental disease, and I think this was their way to make him feel included and normalised. I think the ILBW took this and spun it. He wasn't mad, they were the ones with generational pain. The dimwit wasn't sick, the meds were their way of controlling him, he was sad and traumatised. I think she took him off his meds and doctors that helped him and gave him to charlatans. I also believe that this is why Charles, and even William - despite the planting on the news - are hesitant on being ruthless towards the Dimwit, as they know how fragile he is and they fear what will become of him once the ILBW leaves him - as the whole world knows she will.
I have no proof of this, this is just my opinion, but if you put the facts together - and what we know now about Diana's mental health - I don't think this is a crazy theory.