Wilhelm His name is William Jennings Bryan (no "t") and it was the Gold Standard he was against (Cross of Gold speech), hence "yellow brick road". The main characters all represent parts of America. Dorothy is the common person, the scarecrow is the farmers, tin man is the steel industry and the lion is actually WJ Bryan. Emerald City is D.C. the "wizard" is the president while "the man behind the curtain" is Congress. (They both claim the can give you everything but are pretty powerless. Remember all four main characters had what they were looking for without the help of the wizard.) The cyclone was the dustbowl. The wicked witches can be several different people, most likely the super rich who "enslaved" others. The good witches are probably Carnegie and other rich people who were also philanthropists.
Annie There is a theory out there that book was an analogy/allegory of the Silver standard controversy happening in the late 19th century. The wizard represents William Jennings Bryant and his ilk, the emerald city is Washington DC. In the book, Dorothy's shoes are silver, not ruby, and represent the nation's silver, while the yellow brick road represents the Gold Standard. There's a lot more to it, I'd recommend googling it if you're interested.
Steven Davis L. Frank Baum posited himself as the Royal Historian of Oz. Noted in the Annotated Wizard of Oz, there is evidence in the books that points to Oz existing on Earth as part of an unknown continent somewhere in the southern Pacific Ocean.