Hi, is it 'who are you' or 'whom are you'? 'Who is it at the door' or 'whom is it at the door'? Since the answer is 'I am him', shouldn't accurate usage be 'whom are you' though popular convention says to simply go with 'who are you'?

Who is the subject of the sentence in both of your examples. It should be who. Here's more on who and whom.

If the answer to the question is 'you are him', then the 'who' will correspond to the object ('him') of the answer and so shouldn't the 'who' be a 'whom' instead? Compare the two separate examples below to see a simple illustration of this.

By whom will the woman be saved?
The woman will be saved by him.

Whom are you?
I am him.