It was done with multiple professors and administrators from department. It has changed since I started, and the experience for incoming graduate students is more personalized. It is now a longer event and you will be also somewhat interviewed by other students so treat the entire experience as an interview.

Mainly ask about your previous research project. Also try to connect your project with the project the professor is interested in. May also ask basic ML knowledge according to your target professor.

Prettyy good. Depends on who you get. Most professors are good and kind. So nothing to worry I think. Just be ready to talk about undergrad experience and research experience.