...but in thinking about it - who actually could sue me? I mean Viacom can't lay claim to the content because it has The Thing and Spider-man in it. Marvel can't lay claim to the content because it has Doc Savage in it. To send me a cease and desist, wouldn't one or the other have to have sole ownership of the content?
Not really, they only need to have a stake in ownership of the material to take action against you, afterwhich they may or may not contact the other owner, who could join-in on whatever legal action is taken. The only drawbacks in co-owning something is that all owners of the material have to "sign-off" on anything that is done with the material, such as reprints or adapting the work into a book, film or tv show.
I mean doesn't trademark infringement only cover treading on someone's property in such a way that it denies them future revenue from said property?
Trademark Infringement is when you cause confusion in the marketplace, such as trying to sell comic books with the name/image of "Spider-Man" prominently featured on the cover without an agreement from Marvel to do so. Consumers would assume that the comic was from Marvel or at least authorized by Marvel, and as such, would have a certain level of quality. You'ld be making money off of the reputation that Marvel's worked so hard to establish.
Hosting Copyrighted comic book files for free on your site has less to do with Trademarks and more to do with Copyrights. Hosting Golden Age Daredevil Comics on your site is no problem because the work is Public Domain, so Marvel isn't going to bother you since they don't own that material, even though they do own the Trademark on the name. If you were hosting Silver Age Daredevil stories, then Marvel could take action against you because they do own the stories from those books.