Rationalizing the misuse of the pronoun "it"

I have three inquiries--both reflect similar issues.

First Question:
"It:Third-person singular non-human or inanimate, or impersonal" (wikipedia.org).

We shouldn't use the pronoun "it" when referring to a person; however, we say, "Who is it?"  Is the question grammatically wrong because you're combining "who," acknowledging a human presence, with "it," converting that presence into a thing.  The question should be, "Who is there?" Right?

Possible reasonings:

1. "Who is it?"  At this moment, you're unaware if it's a human or not?  BUT!  If that's so, you wouldn't use "who."  You'd say, "What is it?"  You could say that you're using "it" in its neuter form...but "it" is still for animals. 
2. 1.  Similar thinking: When people say, "Is it a boy?" it may be offensive because it's impersonal, but it's accepted because "it" stands for a gender neutral thing.  After knowing the answer, you switch to gender appropriate pronouns.
3. Using "it" is just idiomatic.     

Second Question:

When someone asks you, "Who is your teacher?"  Why is "He is Mr. Smith" incorrect or strange?  

Possible reasoning:

1. The question is gender neutral.  Answering with a gender is prejudice.  If the question included a gender ("Who is he? Who is that man? Who is your brother?") it's ok. to specifity the gender in the response.  Is this the correct clarification?    

Third Question: 

Is it o.k. to answer the question, "Who is your brother?" with, "Matt is he."  Why does this seem awkward?  Is it just not commonly used?  It's grammatically correct, right?  Subject -->Verb -->Subject Complement.  Is it o.k. to answer with "He is Matt."  This sounds strange, too.  Why do I want to inform the person of their name or use the same pronoun in the question? ("His name is Collin" or "Collin is my brother.")

Thanks a bunch!  I've been broading over these questions all day!

Chris Cimino

ESL teacher in Japan