Charlie: Minhagim can make things Assur. Of course they can. Minhag is a powerful Halachic tool and consideration.
Also, your second paragraph is in theory not incorrect. However, it takes incredibly broad shoulders to say that we can go like Rishonim against the Psak Hamekubal in Shulchan Aruch and Achronim. Shoulders far, far broader than anyone who has supported this issue has.