Hey! So I had an interview for a cognitive program (and 2 more coming up) so things very well might differ from yours! I know clinical programs are VERY competitive (when I interviewed, the Cog program had 7 interviewees for 5 spots...the clinical had 41 for about the same number. Though this was for a PhD program).
These are my own experiences. They are by no means necessarily the norm. Just wanted to give you some of my own insights!
1. I would know a little bit about each. 15 is a lot of people (I had to know 7). Obviously you should focus mostly on your POI, but showing an interest in other faculty members' work shows that you could potentially be a collaborator and so they may see you as a really good fit compared to someone who wants to work with one person and one person alone. You don't have to know all their research, just a basic idea (for example, I knew one of the faculty members does working memory and auditory processing. While I do applied long-term memory, I took the time to discuss her research interests with her and mentioned how we could work together).
2. I didn't bring any, because I was under the assumption that they had all already read my CV. I guess 15 might be a good number so each faculty member can have one if they request it.
3. Applying to a Master's program, they may ask "Why a Master's program and not go straight for a PhD?" Saying you didn't think you could get in to a PhD program isn't what they are looking for. I went to a Master's first, and it was because I wasn't 100% sure what small area I wanted to get in to so it gave me the time to find what I wanted to do and better prepared me for a PhD program, so I knew if I went on to a PhD, I wouldn't quit...because I know how stressful the research is!
4. Honestly, ask them what they like about the program and what they DO NOT like. I've had graduate students be very honest with me about what they did not like about their program, and that is important. Ask how the cohort sees each other. Is everyone cooperative or is it every man or woman for themselves (an environment I could not work in). How do they manage to survive on their funding etc.
5. Nothing I can think of off the top of my head. If you ever forget a question, you could always e-mail the POI or their current grad students (I mean, most extend that offer haha).
Hope this helps you somewhat! If you have any more questions, feel free to ask!