1. There is no law of physics that says that different dimensions have to look totally different from Earth. In fact, real life scientific theory even contradicts these fans' logic. String/M theory, to keep it simple, suggest the existence of many alternate universes; many of which bear nearly identical resemblance to our own universe with slight changes that result from the principle that every possible thing that can happen does happen.
For example (and forgive me if I over simplify this), the theory suggests that if you go into Crispy Cream Doughnuts one day and choose the jelly doughnuts over the chocolate doughnuts, then in another universe, an alternate you may go in and choose the chocolate ones over the jelly. Another example: You may survive getting hit by a car in this universe, but an alternate you may die from it in another. Essentially, whenever a situation has alternative possible endings, all of those endings take places in different dimensions.