Dot's Eyes listed all their suspect behaviour in great detail. If you are in denial it doesn't make you lie, evade, not co-operate, boast about switching hairbrushes and toothbrushes, rail at all and sundry, blame everyone but the "perfect child", etc. By any stretch of imagination or logic, this makes no sense. If they were merely in denial, then why be unco-operative and deceitful? One does not follow from the other. If they believed their daughter to be innocent, why are they not doing everything in their power to prove she is innocent and find the killer?
Denial may be a defense mechanism but to use it as an explanation for all behaviour is to present it as irrefutable and, that, IMO, makes it spurious. If you're going to place all their behaviour under the rubric of denial then you would have to address the different qualities of denial- denial of fact (lying), denial of responsibility (blaming), denial of effect, that is, their behaviour. For example, denial of fact is not simply a defence mechanism.
If you want to take denial to its most radical conclusion you could argue that the perp is in denial as well and lies pathologically because she can't handle the truth. IMO, denial is too simplistic an explanation, and definitely not in keeping with its original psychoanalytical meaning. It also ignores freedom of choice, morality, ethics, personal pathology and family dynamics. Denial doesn't automatically mean that you lie and deceive and defy. It simply can't explain away everything.
Also, there is no evidence that the As ever made an active effort to find Caylee. They just talked about it a lot and sold T shirts and had a billboard in their front yard.