Burning the bodies would account for the other bones they found.
It seems that several of the boys/men knew each other but they were not all of one circle. I don't think they all knew DiNardo. A cell phone ping led to the DiNardo farm and cadaver dogs found the gravesite. Those dogs must be really great since the hole is 12.5 feet deep!
I hope that the plea deal precludes any mental illness defense - they are saying he agreed to "plead guilty" to the four homicides and told them where the bodies were.