1. Don't go too crazy here. Sitting in front of a computer in a suit and tie is a bit of an overkill and many McKinsey interviewers of my candidates for instance recently asked candidates to dress down so they feel more comfortable and the situation is less staged during the virtual interview (and that is during the interview already, not even during a recruiting event). I'd go with a nice shirt and that's it.
2. Think about genuine questions you are interested in. Don't think about questions that you think sound good or may impress the interviewers. Be authentic, curious, and engaged during the event.
3. Referral might be possible by showing engagement during the session, i.e. asking questions, answering questions, going into discussions with consultants. If it's a smaller event, they might take notice and proactively contact you and ask you to apply (which counts as a referral). If it's an event with many more people, you can reach out to one of the consultants you met after the session and ask about application advice (indirectly about a referral).
All the best!