Gary: a belated welcome! I think I might have said hi to your post in the other thread, but I ended up rambling all over the place and deleting that post, so....
You make some good points here and in the other thread. I have a couple of things to say in response.
Prater: there is some confusion about what he was doing on that fateful trip. A lot of news sources describe him as Murphy's legal advisor, but his hometown paper, the Chattanooga Times, says he was representing Modular Properties, the outfit Murphy was looking to invest in and whose plant they were all supposed to inspect. He may well have been friendly with Murph or helped him with legal matters, because his background makes him sound like Murphy's kind of people: semi-orphaned, ex-Army with time in combat, fellow Democrat. Interestingly, he seems to have been pretty meticulous about checking in with his family; Prater's wife was one of the first to become worried about the plane's fate when she didn't hear from him.
On the not-an-accident thing: there's certainly not a lack of people with motive, but there is a lack of evidence of method. Shirleyjean speculated about altimeter tampering up-thread, but that's problematic to my mind, because it seems to require a pilot who's not good with instruments (like Butler), who ignores the altimeter during the initial startup, and then becomes dependent on it sometime during the flight. That is, it requires a pilot like the one the flight had, starting out in good weather where he didn't need the altimeter, and then finding himself in unforecast
bad weather, where he does need it. Not to be sarcastic but I'd like to see a theory of howdunnit that doesn't require the perpetrators to have magic weather control powers.
EDIT: I guess I owe Don Graham an apology, with regard to him glossing over that bit in the TIMES obit. Looking at Murphy's FBI file, it seems like the Los Angeles DA's office was referring to the Buccieri case when it described him as a "source of intelligence" on the mafia, and Don Graham certainly covered that at some length, though he is perhaps unfair (or uninformed) in ending the story by saying that Murphy "couldn't deliver" Buccieri. The FBI certainly seemed to regard this whole affair as a fruitful line of investigation, and the DA's office was all lined up to prosecute three or four REDACTED someones but decided to close the case when their key witness (Murphy) died.