This is actually kind of ironic since "novum mundi" in turn is grammatically correct, although it still does not mean "new world". Latin can be quite a fascinating language, but admittedly also difficult at times.
You are right that "novus mundus" is also correct. Due to the agreement of the constituents, sentence structure is pretty much unrestricted in Latin. Yet usually adjectives as well as genitives are placed behind their antecedent, which is why I have suggested the term "mundus novus". What does actually militate against the acronym PWMN? Since M is basically only an inverted W and vice versa, it might provide an optical advantage to have them both in the middle of the word. However, the decision is of course totally up to you after all.