"Whom" is a legitimate word; however, in my experience the use of whom has been largly abandonded by younger generations. Even in college writting classes taken decades ago, it was suggested to avoid the use of the word "whom." The average person will spend more time wondering if you said it correctly and be distracted from what it is that you are trying to communicate.
No - even though "whom" has not been used much for many years, it would still have been incorrect in this case if we still used it these days. You would use it as the object of a sentence (with whom, to whom), but not as the subject like in the sentence "Who is it?" That is effectively what you are saying with "Then who?"
I had an Italian teacher who tended to start every new part of a lesson, say after a discussion on a particular point, with allora. Toward the end of one lesson someone called out "fifty" and she asked what he meant. Somewhat shamefacedly he admitted he had been counting the number of times she had used "allora" during the course of the lesson. She took this in good part and even told the joke against herself to other students. I am not sure if it would be called a pleonasm but it appears to be very common usage