I'll take a crack at answering this.
I don't think U.S. laws about "parody" and "fair use" have all that much to do with how Marvel was able to legally get away with creating the Squadron Supreme in the first place. After all, Hyperion's name was significantly different from "Superman," and his costume was very different. On the other hand, when Marvel's "What The--?! #7" showed the long-awaited face-to-face encounter (and obligatory mass slugfest) of the Revengers and the Just-A-League, that was
definitely a case of "only getting it away with it because it's blatant parody." (The same way Mad Magazine does lots of parodies of blockbuster movies in which they do "humorously skewed" versions of titles and character names and the plot and so forth.)
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But as I understand the way the courts generally interpret the copyright laws, DC does
not -- for instance -- own exclusive rights to such generic concepts as "this superhero can run really, really, really fast." They only own copyrights on specific character concepts who use the name "Flash"
and have the ability to run really, really, really fast
and wear certain distinctive costume designs (such as Jay Garrick's Golden Age costume or Barry Allen's Silver Age costume).
In other words: Marvel doesn't need to
beg DC for permission before it can publish a story with a group of superheroes called "The Great Society" which includes a heroine called "Boundless" who can run really, really, really fast. But if she wore a blatant copy of Barry Allen's outfit, then there'd be a problem. (Likewise, there's no law against Marvel creating a new extraterrestrial superhero, a teammate of Boundless's, who is a member of a
green-skinned species. DC does not own a copyright on the broad concept of "green-skinned aliens.")
Incidentally, it might interest you to know that a character name -- such as the colorful costumed alias -- is not automatically "copyright-protected"
all by itself, either. A character name is, I think, simply too small to really count as "a unique and copyrightable piece of intellectual property" in its own right. (I've heard the same applies to the words or phrases used as titles of novels, movies, TV shows, etc.) Some years ago, I used to amuse myself (and other comic book fans) by doing annual updates of a list of
all the times when Marvel and DC have each used the
same names for different characters who are not supposed to have anything to do with one another.
At last count, I had found
1139 "shared aliases." Examples which have been used on both sides of the fence include Captain Marvel, Huntress, Mockingbird, Oracle, Doctor Doom, Doctor Destiny, Magneto, Professor X, Phoenix, Iron Fist, Ghost Rider, Catwoman/Cat Woman, Spider Girl/Spider-Girl, Kingpin, Spoiler, Mister Sinister, Miss America, Scarecrow, Mad Hatter, Electro, Sandman . . . you get the idea. (And as far as I know, none of those cases of duplication have led to big messy lawsuits!)
Here's a link if you want to see the full results of my research (which included hundreds of helpful suggestions from other comic book fans who liked what I was doing):
Character Aliases that Marvel and DC Have Both Used (5th Draft)