Egomania or something else?
Probably a combination of ego, obsession with a subject and the self validating echo chamber effect.
I've no idea what YH did for a living but I suspect she may have been just an ordinary housewife who now feels important. MDB became a published author for writing such gems as " Justice is never served by convicting the innocent" - a cringing, blindingly obvious statement that she seems ever so proud of and uses like a catchphrase.
I wonder if she says to her husband,
"Tea is never made without boiling the kettle"
I’m certain Michelle Diskin Bates knows her brother murdered Jill Dando
I don't see how you can be certain. Denial is powerful and only increases with emotional investment.
I sometimes amuse myself with young earth creationist videos on youtube. It's fascinating to see how people will deny or ignore known facts and entire branches of science in order to protect their belief system which appears to be a house of cards. If they start to question one aspect of the belief system, they might start to question it all so they simply don't go there.