The precise translation to your question is "Ποιος από αυτούς είσαι;" Which one of the people you see. But yes, in a simple discussion you could also say that.
I am not a native speaker, but usually in languages like this the masculine version is dominant and the feminine is only used when it is clear that whoever is meant is female (or a female only group). However in this case both should be correct in my opinion.
Yes, exactly! There are also the alternative genitive forms: ποιου, ποιας.
Who is this would be "Ποιος είναι αυτός;" Είσαι is "you are", not "he/she/it is".