It is really hard to separate the facts from fiction in this case. Maybe we can pool our knowledge here. I think that most people believe that the mother was involved based on the fact that the handwriting analysis did not rule her out. The problem is that the note was written with a Sharpie and one of the analysts looked at a photocopy and not the original. The note itself contained many quotes from violent movies.
Most of the evidence actually points towards the father or one of the father's friends or business associates. One of his friends had a wife who had written a novel about a girl being strangled in a basement. All of the friends were questioned and VERY quickly cleared. I think that there have been major payoffs and that the case will never be officially solved.