It's been a while, but I continue to think about this case, since I did know Linda Mayfield (Sohus), though had not talked to her for years before her disappearance. But here is what I think happened:
Linda took the cats to the boarding kennel. She couldn't afford to leave them for any longer than necessary, so this would have been the day before they were scheduled to fly to NY, or the day of the flight. When she returned, John was still gone doing whatever errands that he needed to do. Linda got into an altercation with Chichester. Maybe he 'came on to her'. Some reports have implied that he was rather forward with women, and Linda's best friend says that she had mentioned that Chichester was "creepy" and she stayed away from him. If he had made advances to her, she might have pushed or slapped him, and he could have accidentally killed or injured her. Or, she may have caught him stealing from John's mother, or doing something that he didn't intend for her to see. Anyway, for whatever reason, he ended up having to dispose of Linda. I think that he took her to the wilderness and dumped her. That area is surrounded by wilderness, a National Forest. It would have been so easy to take her up to the mountains and dump her. Then, he would have returned to the house, and he would have KNOWN that he had to kill John. There would be no way to convince John that Linda was all right, so John had to go. However, since Linda was already dead, Chichester had time to dismember and bury John. To me, that makes the most sense considering that Linda's body is not with John's. And I don't think that Chichester is a killer by nature, or he would have killed his recent ex-wife, since there was much more money at stake, as well as his daughter.
Anyway, I bet that Chichester stayed around for months for two reasons. One, to make sure that the police weren't digging in the yard. Two, to turn a bad situation into a profitable one. I BET that if we could look at John's mother's financial records, we would find that Chichester was getting money from her, to send to John. He probably told her that John and Linda got the job in NY, and were sent immediately to France. That they needed funds, since there wasn't even time for them to come home and get their belongings. That only he, Chichester, could send the money to John because the CIA has sworn everyone to secrecy or whatever. If we consider that John's mother went to her grave convinced that John and Linda were in France, and that she went bankrupt eventually, it is very likely that Chichester convinced John's mother that any money that he was given, would get to John. There are hints that Chichester even went to France... he probably wrote the postcards from there. Who paid for the trip? John's mom, that's my bet. If the police could look into whether a large amount of money was withdrawn from her account in the months following the disappearance, they might be able to tie Chichester to the murder that way.
When he was sure that the police weren't looking into the case, (after all, adults have the right to disappear if they want to),and when he had gotten as much money as he could out of John's mom, it was time to move to new grounds and a new name. So, he took the pickup and headed out. But he no doubt kept tabs on the mom, possibly contacting her periodically to keep her quiet and to know whether any investigation had started. It would also explain the phone call to the book store... the 'inquiry' into whether Linda worked there. If the bookstore owner had reacted with curiosity about Linda's whereabouts or any hint that the police were looking for her, Chichester would have hung up the phone and known to lay low.
Chichester isn't by nature a killer. Something happened to cause Linda's death, something that he didn't foresee. He panicked.
If we could explore any info on skeletal remains found in the mountains above San Marino in the years after Linda's disappearance, there might be some DNA that could link found bones to Linda's sister.