Hi there,
1) Is it worth adding a line saying that you've done X number of cases with people from different countries, or should case prep not be referenced at all?
No, because what matters is if you can solve properly the case they give you, not how many cases you have done in the past. Additionally, given they take for granted you prepare before the interview, I agree with Alexandre that could give the impression that you are not confident about your level of prep.
The goal of the CV is to show you have proper skills, experiences and achievements, not that you have prepared for the interview – that will be evaluated during the interview process itself.
2) Is it fine to mention in the interests section how many countries you've travelled to / lived in, or would it come across as bragging?
It depends on how you present it, if properly reported it could be fine. It also depends on the “opportunity cost” – what else you would not mention instead of that due to lack of space.
Good luck!
Francesco