What was all the slapstick comedy about?

Well, they went with a different director for this one. Richard Lester was known for a bunch of comedies in the sixties and later he did that two part Three Musketeers film series in the seventies which also had slapstick elements thrown in.

"For instance, when Gus takes control of all the (connected) computers in the world and they go haywire, a man sees his credit card bill and puts a grapefruit in his wife's face. That is directly lifted from the film The Public Enemy (though The Public Enemy isn't a comedy.)"

So? The man obviously thinks his wife has been overspending far too much, seems a reasonable reaction to me. I would've divorced her, personally, which would've made it funnier.

"In one scene, Superman turns a lump of coal into a diamond. That is, or was, a frequent part of Superman comic book stories. Of course, this is because both coal and diamonds are carbon, but have a different atomic structure."

In that case, Superman III is paying homage to the comics. It IS a comic book movie, after all!

The rest of your points are irrelevant, it is comic book science, which never makes sense most of the time. The MCU movies do it all the time as well, dumb as THEY are.

The OP obviously never watched The Man From UNCLE, if s/he thinks Robert Vaughn can only play antagonists.

I will never understand what everyone involved with this film was thinking. Despite the success of the first 2 films, the studio executives pushed for this to be a Richard Pryor comedy and made Superman a supporting character in his own movie. I don't care what anyone says. This is worse than Superman 4 and Man of Steel. Actually, I like Man of Steel but this movie is the epitomy of how not to make a Superhero movie. It's boring except for the scene of Clark Vs. Superman and the scenes with Lana Lang. But everything else in it is just stupid and boring.

“I know part of the history of comic books is to use intentionally bad science as part of the storyline”

No truer statement has ever been said.

As for the diamonds vs other forms of carbon, it is the same carbon atom but the crystal lattice structure that describes how the individual carbon atoms are connected together is different. The same way you can make a lot of different structures with the same identical Lego pieces. I’ve always heard that diamonds are formed when carbon is submitted to extreme heat and pressure so I guess that is how they justified that scene.

I have no idea. I thought "Superman 3" was really dumb. I was a teenager at the time it came out. It was a real let down from the first two.

You really have to understand that before the 80s comic books REALLY WERE kind of stupid. They were "junk" for kids, or at best a source of camp like the 60s Batman TV show. And that was considered ok! It would've been "missing the point" if you made a comic book movie and took it too seriously.

Now people were indeed down for the reverent tone of Superman I & II and they're in part responsible for comics (eventually) getting a classier reputation, but the long origin scenes of the first were almost unanimously considered the worst part of both movies, going too far and considered too dramatic and pretentious. The second half of part I and the majority of part II, being lighter and more comedic, were praised as being "more true" to the source material.

So they double and tripled-down on that with Superman III, and the reaction to that is you definitely don't want to go too far in that direction, either!

As the years went by and people had less and less of the idea that comics were just "trashy junk for kids" (largely because comics themselves had grown past that for a long time) people were better able to take the reverent and dramatic long first act to Superman I at face value. Superman II and especially III aged kind of poorly afterward, as it appears there's this long trend of the films getting steadily sillier and further away from the tone set by the beginning of the first film, rather than it looking like it was the beginning that was betraying the tone of the rest of the films.