"Is it a girl?"
Sorry, in most cases the only way to tell if a statement is a question in Russian is either by the question mark, context, or if the speaker raises the pitch of their voice at the end like we do when we ask a question in English.
Also "Этот девочка" wouldn't work, "Этот" is masculine while "Девочка" is feminine. The two either have to agree to form "Эта девочка" -> "This girl", or you use the particle "Это" to mean "This is".
Not in this case. It will sound as if you are calling this girl "оно" ("it") which is kind of offensive. You can say "Она девочка?" (Is she a girl?), but this is not a direct translation of the given sentence.
Это is a demonstrative pronoun, it can be used with animate or inanimate objects regardless of their gender and number.