It’s hardly a vast “conspiracy” when you consider the culture/loyalty that waldojabba mentioned. And besides that, Nearly all of those in attendance at the Albert home that night are tightly bonded/intertwined as family, iirc. Add the lead investigator MP as a close friend to that group. These people have lived intertwined lives for a long time. There are at least two generations involved in these families, too. Some families will do anything to protect their own.
Those who weren’t family but just friends, appear to have no knowledge of the wider going-on’s that night. So cross them off of the so-called conspiracy list.
I think it’s worth remembering that any first responder could be indirectly intimidated by this seemingly powerful group in that town. I can’t remember if we had any issues with testimony from EMTs, for example, though. In some places, you keep your mouth shut if you do happen to know something, too.
I just don’t think it’s that hard of a concept to grasp.