How do you know for sure that the LE hadn't investigated or at least inquired about what happened to Max? Plus, if the LE had already declared Max's fall to be an accident without even bothering to investigate it, why would Rebecca have killed herself then? And if she truly was responsible for what had happened and killed herself because of her guilty conscience, why didn't she write an apology or confession admitting that she was the one who had caused Max to fall from the stairs? I know that this is thread is about Max, but it's hard not to talk about Rebecca as well since the two deaths are linked together.
Even if the scene of the accident had been cleaned days before anyone had decided to look into Max's demise, that still doesn't mean that the LE wouldn't have been able to glean information as to what exactly had happened when he fell.They could have interviewed Rebecca's sister, and if her statement had contradicted Rebecca's, they would have or should have taken note of that and investigated it further. And I believe the LE have also examined the area from where Max had fallen from, there were a few photos back in one of the older threads which showed the police using a blue light to examine the walls and the staircase of the house. Plus, they would still have known what the original scene of the accident had looked like based on the initial police report on Max's fall. The ME was the one who had determined that Max's injuries and subsequent death were consistent with those of an accidental fall, this is the same ME who had also declared that Rebecca's injuries and death were consistent with a suicidal hanging rather than a homicidal one.