Do searchers ever "work outwards in"?
Asking because while I understand it could be counterproductive (starting at the outside of a circle drawn 5 mi. in the desert and working towards the RV/center, does mean that by the time the searchers reached the nucleus they could be extra tired and miss things); I'm just thinking, I know that sometimes they tell you if you're working on a paper, you should read it backwards line-by-line to get a different perspective.
The problem for me is, how many of those desert-disappearing people had good known reason to disappear? The example you provided, that of Charles Manson's former attorney, is a case in point - he was responsible for psychopathic Manson going to jail. Also the reason why everyone assumes that Jimmy Hoffa will never be found alive - because he was killed by the Mafia, and the Mafia is good at hiding bodies.
I can't help but think, there's a difference between the ease of finding somebody who's trying to disappear and knows they're trying to disappear; and somebody who had no intention of going missing in the desert. Like the difference between playing hide-and-seek with a toddler, who just drops where they lay/in a place not well-covered; and playing with a college student. The college student's goal is going to be disappearing, albeit temporarily; and they are probably going to know how to achieve it. Your average toddler, not so much.
I don't doubt that a large amount of people have disappeared into the desert. What I do doubt, is that every person who has disappeared into the desert never to be found, is the type of person about whom you would say "Why no, he/she had no enemies that I could tell you about." Therefore, I find it hard to believe we can draw any hard conclusions en masse after the fact about "people who have vanished in the desert", without knowing whether or not the people in question got themselves lost purposefully and with planning ("starting a new life"); had enemies who might have, shall we say, "pushed for" them to be lost a la Tony Soprano ("Did he/she have any enemies?"); etc.