aley it's a bit complicated right now. It will be a lot easier to manipulate login/signup fields in beta8.
You shouldn't replace the
body function with your own that way, because it will break all other extensions trying to do the same.
Instead if you want to modify the DOM you should do the same as the Flagrow Terms extension does. Extend the
body function, retrieve the original virtual dom and manipulate it. You will have to browse the virtual dom to find items you want removed and delete them from the virtual dom.
Or just use CSS/jQuery to hide the fields you don't want and use events or middlewares to block classic login server-side, way less complicated