Hey Kadmos1,
I just wanted to re-open this can of worms for you, on what I am doing about the subject myself.
I am also writing a novel called Greystoke & Man Cub. So, here is the deal, which I have found, to work around it. First things first: Tarzan is public domain. Nothing EBR, Inc. says or does can change that. I understand that when trademarking a characterization of a public domain person, it is the “brand” of the character which is trademarked.
For instance, did you know Dracula is trademarked? His image is. Which is why they need to change it up all the time when new films come out. Universal does not want any other film to have the features which Bela Lugosi’s version has, cause that look is trademarked.
Another way to view a brand, is to not promote it on the face product, nor to refer to in in promos or sales. In other words, you can have Tarzan IN your book, but not on the cover nor in the synopsis, or sales pitches/promos. Characterizations must be within the product, but not outwardly, as a means to “make the sale”, so to speak. This, of course, can be easily done with the right verbiage.
If you want to play it safe even more; use Tarzan’s other aliases. Lord Greystoke, John Clayton, etc. In my novel, I plan to never once use the name Tarzan. I will use his alias names, which are not trademarked (apart from Greystoke, back in the day, which was for a cigarette brand, but it totally legit to use). I don’t even plan on using any of the other trademarked titles which ERB, Inc. has claimed, including Jane (which yes, she IS trademarked).
I understand what you are asking in regards to their trademark vs pubic domain, and it is a sly and snaky way of trying to perpetually keep a copyright. But, after 100 years, Tarzan is in the public domain. They can try to keep him out with this backdoor move, but it will not work. Those tales now to the public; that is the law of it.
Even so, they still are trying to make it impossible, for their own gain, to keep him away from fan fiction; but there are ways around that. Truth is, you do not need to use the specific name of Tarzan, go around it by other means, if you wish to play it safe. That’s what I intend to do. They cannot take you to court for using a public domain character name, which is an alias of a trademarked brand.