So the questions are:
a) Why would they do this?
b) What was the payoff?
c) Why would they keep mum about it for forty-something years?

Well, it happened at a period in their career when Paul was becoming the breakout star in the band, and shortly after the whole “Bigger than Jesus” debacle. I’ve heard people suggest that the whole “Paul is Dead” thing was part of an elaborate gag John cooked up to prove they really were “Bigger than Jesus” by “Killing” Paul off and “Resurrecting” him at some later time. They dropped clues in to the album, the occasional subliminal message, that kind of thing. We got as far as them “Killing” him off and him being “Replaced” by the alleged imposter Billy Shears, but then the whole thing was simply and abruptly dropped, the “Paul is Dead” references more-or-less disappeared, and that was that.

If there was a plan to kill and resurrect Paul symbolicly, what could have happened to make them abandon it so abruptly?

Well, this was right around the time of the Manson murders, and Manson was pretty emphatic about how the Beatles were the Four Horsemen of the Apocalypse, and how he was getting ‘messages’ from them. Now, if you’re the Beatles, and you hear on the news about how that super-hot Blonde chick from the final Matt Helm movie got brutally murdered by a crazy person who claims he’s getting secret messages from the Beatles, it’s not going to take long to realize that he MAY have been picking up somewhat on the real, actual secret messages you put in there as a joke. So you recoil in horror, and shut the whole thing down, and never admit it to anyone.

There you have it.

I didn’t know Dean Martin was on the list, but it could easily have been not because of Sharon Tate but because of Dean’s son Dino. Dino was in the Hollywood Royalty Boy Band Dino Desi & Billy (Dino son of Dean Martin, Desi son of Desi Arnaz & Lucille Ball, Billy Hinsche son of a major Hollywood Real Estate playa), and on the basis of blood ties and marginal musical talent were able to have a successful career and maybe that made Manson jealous or something. Who knows.

You mentioned the movie The Wrecking Crew; as another weird coincidence, the group of elite studio musicians who always were first called for Brian Wilson, Terry Melcher, and groups like Dino Desi & Billy, were known as “The Wrecking Crew.”

It’s my belief that laughing at a monster is a good thing. It diminishes the fear that they hope to engender. We make fun of Hitler all we want. We tell Stalin jokes around my house all the time (Which started when The Tick cartoon in the 90s started telling Stalin jokes). I think we all can agree that Stalin and Hitler were far, far, far worse than crazy ol’ Chuck, so why should he get a free ride when the biggest monsters of the 20th century get mocked by me and mine?

Comedy is a defense mechanism, a way of coping with things that scare us. We laugh at them, they get less scary. We laugh at them some more, they get even more trivial. The added bonus being that turning monsters in to punch lines for jokes keeps them from having that eventual transformation from antichrist to misunderstood tragic hero that you get with, say, Napoleon.