Trademarks and copyrights are two distinct things in law. You can't copyright a name, but you can trademark it. Fawcett retained the copyright to Captain Marvel until it sold the character and related properties to DC. What it lost was the trademark to the name "Captain Marvel" as a title for publication.  

DC can still use the name "Captain Marvel" for the character to this day, if not for the book or movie title. However, after nearly five decades of not being able to trademark the name, since Marvel has held the trademark since 1968, DC finally decided to go ahead and just call the character Shazam.

Copyrights have a specific shelf life. Trademarks are more of a "use it or lose it" situation.  Marvel lost its trademark to "The Champions" in the 1980s because it couldn't prove successfully in court that there were plans to use the title for a publication anytime soon. So, Hero Games (I think that was the gaming company), won the rights to the trademark.  These days, Marvel seems to have gotten back the trademark. 

And that brings up another thing: Once a copyright expires, the property enters the public domain. Trademarks expire,  but can be renewed even years later.

The reason Marvel published the Captain Marvel specials in the 1980s was to retain the trademark.  The publication trademarked doesn't have to be in constant publication, but there has to be intention of using it within a given period of time. 

One more thing about the original Captain Marvel and the arrangement between DC and Fawcett in the Seventies: As I understand it,  DC was originally licensing the character at that point, and didn't fully own the property until the Eighties. Does anyone here know the full scoop on that?

The only version of Captain Marvel that Johns ever worked with was the post-Legends "naif-in-a-man's-body."

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That's not what I remember at all from Johns' JSA issues with Captain Marvel.

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It was during Johns JSA run that he started introducing the idea that Captain Marvel was the superhero version of Tom Hank's character in the movie Big. If there was a case for why fans shouldn't become pros, Johns would be the prime example of why fans should not become pros.

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That’s not what I recall. From what I remember, Johns was following the cues in Ordway’s run where Billy and Captain Marvel were the same person, yes, but he had a more mature personality as Captain Marvel. The only instance where I recall where Billy’s youth was an issue was when Stargirl discovered Billy’s secret identity after an anti-magic field reverted him from Shazam in front of her. Stargirl and Billy were kind of dating, and the other JSA members noticed their chemistry while he was Captain Marvel. Either Jay Garrick or Alan Scott told him to knock it off because he thought he was too old for her.  But all the JSA members thought Captain Marvel was someone that looked his age and not a kid in adult’s body. That didn’t come until New 52.