Ryujin
Legend
There have been more than a few lawsuits, in recent years, about "work for hire", most notably in the comic book world. Some of those came from the estates of creators, as the creators themselves had passed. Things can be rather murky and I mention Ellison, specifically because he doesn't shy away from lawsuits.You mean because The Guardian of Forever appears in the Animated series' best episode? The Guardian also showed up in Discovery recently. I think, unless there was something weird about HE's initial contract, as work for hire, all rights indisputably belong to Paramount.
As for the actors, as far as I know, only three of them are still alive. Which is the case of most of the stuff from the 70s and earlier, anyone who might have an issue is busy being dead.