So according to the Public Domain Wiki, Mary Marvel (and Captain Marvel) are characters that fall into the public domain. Here’s the tricky part, DC Comics own the trademark to Mary Marvel and Shazam while Marvel own the trademark for Captain Marvel. What does that mean? Well, I’m not a lawyer but on the Wiki page it says that you can use the character, but you cannot use the words “Captain Marvel,” “Mary Marvel”, or “Shazam” on the cover of a publication.
So, when or if I ever get around to making that Real Superheroines webcomic, Mary Marvel will definitely be living in the mansion. Since webcomics don’t have a cover, I’m not sure how the trademark law applies, does that only mean that I can’t use their trademarked names on the homepage? Who knows.
Anyway, here’s the process gif.