The term mature student may have different connotations in different areas, but I've always known it used as an undergrad student who's older than the average due to either starting late or coming back and trying again. In my case I did some undergrad after high school, 1995-2000, and left before completing my degree for health reasons. I then came back in 2010 and started over. I did four more years of undergrad and then was accepted to vet school. Still don't have that undergrad degree.
In the gap from 2000-2010 I was lucky enough to work a lot of jobs that gave me animal or veterinary experience in some way, so that helped a great deal with my application.
UCVM has you sign confidentiality, so I'm not comfortable saying too much about specifics of the questions. Plus the questions change every year anyways so telling you what we were asked probably wouldn't be helpful. I'll just say that most of them are complex, and require you to consider many different perspectives in formulating your answer. The ethics book by Bernard Rollins (the one you mentioned in your first post) is very helpful for preparation, as you've already heard. I think they're looking more for how you reason through the question and come up with your answer and back it up, rather than looking for any specific answer to a given question. For UCVM there is also an essay, and again I think they're looking more for your logic/thought process and how you articulate yourself rather than a specific answer to the question.
Are you familiar with how an MMI works? It's very different from what most people think of when they hear "interview."