Maybe it's just trademark, but there's more potential for a lawsuit, since the two Cat-Man characters look so similar. Accidentally take a step too far in the wrong direction, and the issue is now Substantial Similarity. That is, even if your elements from exclusively public domain sources start to ape something under copyright (say, Fawcett's Diamond Jack learns he's part of the Diamond Jack Platoon, with Diamond Jacks from every Stellar Precinct as set forth by the Commissioners of All Space), there's gonna be some 'splainins to do.
As another example, figure that using random Quality characters on a modern superhero team is almost certainly OK, as long as there's no chance someone will assume your book is published by DC (which is what trademark's all about). If that team happens to be Uncle Sam, Doll Man, Black Condor, the Ray, Phantom Lady, and sometimes Firebrand, you're probably right on the border, even though you're not taking anybody else's copyrighted idea--neither the characters nor a roster can be protected. If the early Star Trek episodes are in fact public domain (it's a surprisingly close call that needs a lot of definitions hammered out), and you bring these Quality characters into action in the timeline where Edith Keeler's survival allows the Nazis to conquer the world, you're probably living dangerously close to the old Freedom Fighters series, even though you've authenticated every element as coming from a public domain source and, depending on the judge, even if you could somehow prove you knew nothing of DC's history with the characters.
As for good faith, a judge could use it to wipe out the fines (which is what I was getting at, in terms of keeping track of the research and going as deep as possible), but it's not going to get the case thrown out, and a malicious plaintiff (the real danger, in my eyes--big companies probably play straight) can still drag the suit out until you settle or run out of money. Generally, being unable to continue the case is a loss, as far as I know. In the end, it's an "ignorance is no excuse for breaking the law" kind of situation.
Overall, though, the fact that there's any documented proof of copyright protection, even if the only copy is in the hands of the owner you couldn't find, you still didn't do "enough" work. The assumption, now that copyright has become about "rewarding the artist," is that you're supposed to wait for permission from the artist's representation. (Don't feel bad, they want the right of refusal on technology, too.)
