Regarding the notices to Sony, yes, we can infer a lot. You don't ever send a third party legal notices as a courtesy. You send them notices because they're named in the suit. That, in turn, severly narrows down the characters that are of interest to Kirby's heirs: It can only be (well, include) characters optioned for movies.
I agree regarding journalism, though. Even in the "big leagues," it seems like everybody just repeats whatever comes off the AP or Reuters feed, and that's barely researched, either. So when comic book journalists don't get into th details, it's hard to be surprissed...
Regarding the other thread and DC characters, it's important to remember that "characters," as such, don't get copyright protection, and that trademark does not correlate to copyright. Which sounds like a bunch of mumbo-jumbo, but is actually pretty straightforward.
First things first, a "character" is not a standalone work. You know how people say that you can't copyright an idea? Characters are the ideas. All you can copyright are fixed works, the stories in which the character appears--the story "fixes" the character, and THAT can be protected. Later uses are derivative works of the stories. So when someone refers to a "public domain character," they (should) mean a character whose stories have fallen into (or have been granted to) the public domain.
In turn, this means that a character can be considered to be in the public domain as long as you can find a single public domain source and ONLY derive your interpretation from traits that originate in that (or those) public domain source(s). If you derive from a copyrighted source, though, you're breaking the law. In a sense, that means you "need" first appearances and/or origin stories, since everything technically derives from them.
But that's not the entire story, because some companies "own characters," even when the stories have fallen into the public domain. That's because they (generally speaking) bought the rights to use the trademarks relevant to the character (the names and likenesses) in their "trade dress" (the prominent parts of their product to identify what they're selling).
In other words, DC "owns" Mr. Scarlet, so it'd be hard to get away with filming a Mr. Scarlet movie, for example. However, since Butler's stories are almost entirely in the public domain, you could use him as a character as long as his use isn't important to your marketing. Basically, you're not allowed to put DC in a position where it looks to a potential customer like they're involved in or approve of your work unless that's really the case.
As to DC characters themselves (I don't consider the acquired companies to be "proper" DC characters, since they didn't start there), as mentioned, has done a pretty good job of registering for and renewing copyrights for all its books.
There are exceptions, but most people would never know or care. Those books published exclusively by National Allied Publications (Major Nicholson-Wheeler's company) don't seem to have any copyrights, which is where the Dr. Occult reference comes from. The Superman comic strips were rarely copyrighted at the start, though technicallly they derive from the comic book. I'm sure there's an occasional issue, here and there, that slipped through the cracks. (Note that, as a result of the Fawcett appeal decision, DC seems to believe that the Superman strips were retroactively copyrighted, somehow. It's an impossible interpretation of what Learned Hand said, but not a direction for the faint of heart, I imagine.)
But otherwise, you're not likely at all to find a DC character anybody's heard of whose stories are substantially in the public domain. Partly, this is due to having staffers like Jack Schiff, who were avid and able pencil-pushers--and in Schiff's case, I mean that with nothing but respect, being paperwork-phobic, myself. And partly, it's probably because many savvy editors would've been concerned about continuing a continuity when parts of it have fallen into the public domain.
As to Kirby works in the public domain? Sure, but I'm not the person to ask, but I do know that, if you search the main site for "kirby," it does turn up a handful of books, at least.
I hope that's of some use, at least.