"Once more with Feeling" was both a Buffy episode and a Musical first. It works as a BtVS episode because it is emotional. It works as a musical for the same reason. Neither aspect is first because both qualities are first and only.

Morble is a lot like Mombi. Morble could have changed her name after the Wizard fled Oz. Mombi was the main villain in the Second Oz book. Glenda makes Mombi chose between execution and voluntarily drinking a potion that permanently strips Mombi of her powers and most of her memories. Try the idea that Madame Morble has to face her crimes as Mombie in the second Oz book.