I think it has to be SA or one of the Avery's who lived on the property. I just don't think the boyfriend or anyone else kills her on someone else's land, takes time to stay there to burn the body, reposition her SUV, and then spread the remains by moving them to a couple new locations. Anyone who didn't live there would not know where everything was located and would want to get away ASAP to avoid being spotted by one of the residents. The boyfriend of anyone else would stick out like a sore thumb lingering around on the Avery property while the body was burning.
And the only way the remains get spread over three locations is if she's put in the burn barrel starting at the quarry, then dumped off at the burn pit by SA's house, then the barrel is returned to its original location. It had to be a resident doing all that.
There is no proof she was killed on Avery land. In fact, I think it's probable that she was killed somewhere else, and taken there. I get the boyfriend was a long shot in this case, but any other murder case, where the victim has a month worth of stalking calls, and you can damn sure bet the cops are investigating. In addition to the fact the ex, and her roommate, decided to essentially hack into her cell phone records. Also, somebody deleted messages, either them, or the brother.