Mankiewicz's brother said that there was a real sled with the trade name "Rosebud," in their Midwestern childhood, and that's where Herman came up with the idea and the title. To be fair to the new "Rosebud" of this thread, according to the article, she is not quoted as saying she is Welles' direct inspiration. She says, ". . . the sled was named after me." In fact, "Custer's Last Stand," especially after Anhauser-Busch got hold of it, became one of the first pieces of American event/product advertising. There is room for some confusion about how far she wanted to push her claim
Yes, the popular explanation for "rosebud," spread by the simplifiers, who want CITIZEN KANE to be reduced to some petty revenge on William Randolph Hearst, does lay it on Marion Davies. Of course, Mankiewicz might have known the rumored nickname, and Welles could have learned it from Charles Lederer (nephew of the "breezy Marion"), who had married Welles first wife.
Finally, "Rosebud" was just a common term of endearment in "the breezy 20's and 30's."