I believe the cases are totally different. For Max's accident, only thhe CPD investigated, and by the time the Doctors at Rady's had called Child Protective Services, Rebecca Zahau had already sent her sister- the only other witness home - and then hung herself later that night/early next morning. So there were no polygraphs, and all the evidence had been at the house and had possibly been tampered with by Rebecca by the time Max's accident was invetigated (exp., the chandeliar at the trash, and the missing link). So the main POI is dead, and had time at the mansion to get rid of anything that may have been evidence. Max's death was simply what CPO thought happened.
In Rebecca's death, all the POI's cooperated fully and the scene was throughly investigated and solved through forensic evidence with the help of the FBI and Dept. of Justice. Much different scenerio.
I haven't followed the McStay case, but don't compare cases anyway.
The scene was not actually fully investigated by the plaintif's attorneys. The catch-22 that Jonah and the SD sherriff's office gave them successfully blocked them from examining it. Then the remodel further destroyed any possibility of further examination.
Therefore, you could say that "evidence at the house had been possibly tampered with".
Yes, I know you are stating that LE 'thoroughly' examined the scene. But it is interesting that PI's from the Plaintiffs' side were not allowed entry and all possible evidence that may have remained has been destroyed forever.