What evidence are you bringing to the table that they made a mess of the investigation Dave?? Very few people know what really happened to Madeleine and I do not count the McCanns among them.
The abduction from the bed theory is dead in the water according to all the evidence which only leaves the awoke and wandered alternative if abduction is to be believed. If that is what happened it puts the parents firmly in the frame as far a culpability is concerned and that is not libel but fact!!
That is their real fear...that by their negligence they were directly responsible for facilitating an abduction.
Facilitating an abduction isn't the worst. I really don't believe that a potential abductor, going through the car park, saw Madeleine and decided to abduct.
The unbearable is the reason why eventually Madeleine went out : she was scared, needed her parents and searched for them.
The reason for denying could be just this.