My unsolicited theory. Im going to skip speculation on her death, as no idea what happened. I lean towards unintentionally going to far/accident, only because if it was planned, I don't think he would have returned home. I think he would have fled then before anyone was suspicious, but who knows.
Because I like to think he comes from a normal, non-mastermind family this is my theory of what transpired.
He comes home and spins a story of why she is not with him. Based on him being home only a few weeks prior, I assume they knew the relationship was rocky. And they believe him cause it is their kid, why not believe him. Then the pressure mounts.
Run in with a very anxious step father on the 10th, parents decide to get a lawyer cause they think “oh crap something happened after my son left her” or “crap he did something”. pressure mounts more, Gabby reported missing, van taken away on 10th, protesters and media start coming to the house. Constant calls from Gabby’s family, media. They decide to go on a family camping trip to escape it on the 11th ( this has been confirmed as the wknd).
Or they are suspicious and think lets get him out of frenzy and get the real story. On the camping trip he confesses to what happened and they decide as a family, you have to turn yourself in. Its agreed they come back from the trip all ready to turn himself in, he chickens out and flees. They then give him a day or 2 to come to his senses, he never returns, they then call the FBI. Alert he is missing. Gabby's body is then found, and police get warrant for their house.
It is very telling that they were there inside for the search. So either the warrant was just for BL and Gabby’s things and they needed help to identify what was what, or they may have been helping. Another interesting thing is that no lawyer rocked up to the house. At this point the NY lawyer would be amiss to not recommend local and more experienced criminal defence lawyer. So a family very quick to get a lawyer, doesn’t get one for the search? I have a feeling maybe his parents are just as shocked, and upset and in disbelief and maybe just misguided at first, but actually helping LE now . I think he told his parents he would be at the reserve, and he drove the mustang there himself. Whether he is there is another matter. JMO