Regarding anyone knowing the layout of the house, wouldn't any caretakers, babysitters, people living on the property, visitors, friends, and any of the family qualify as well?
I agree with this. And I am not certain that the reason that they were taken to the master bedroom was so that they could be shot off camera. In fact, they already shot BB ON camera (in the leg) so, they were not shy about that.
They MAY have been taken to the bedroom because that is were one safe was. Melanine was shot next to the closet that the safe was in. Maybe they were taken to the master bedroom in order to force the comibnation from them.
So, I do not believe that the fact that they were murdered in the bedroom confirms that the Ninjas had an insider. There may have been one. But this is not proof of anything.