(A) Mundus is the Emperor of all Devils. In the game it isn't stated he owns all of Hell. He has Devils running different parts of Hell, and he runs the largest part I assume. Yet, there are different realms in Hell, and Mundus is not the controller of all of these realms. Example... Berial is the conquerer of the Fire Hell... Not Mundus. So that belongs to Berial. Although I'm sure when we are talking about Satan, he supposedly owns all of Hell *sure that is what is depicted in Christianity (Not a Christian)*
So when we look at it like this, Mundus has no ressemblance to Satan *from Christianity* As we know it.
So Mundus, is in resemblance of a God and not an Angel. So he is most likely NOT a fallen Angel.
(B) And furthermore, in Dmc 1 there are statues of a Water God... So indeed Gods are mentioned, but Angels are not mentioned. And Credo was trying to be like an Angel, yet he was using demonic power... And not Angelic power... So Angelic power is supposedly non existent as we know it.