It’s strange that the interviewer does not have their email listed on the school website. Is it an alumni? They normally leave out their email to answer any questions the applicant may have (and to receive thank you notes, of course).
I was also asked about “other schools I’m applying to” in a couple of my interviews. The interviewer is probably just trying to ask for your commitment to their school. I did not bring up any rival schools, because I heard that some interviewers, especially alumni, go absolutely crazy after hearing that. Also, after I listed out my schools, I followed up by saying that “school x is my top choice because…”, kind of just shows my fit and commitment toward the school.