First, welcome to the preplounge platform. Always happy to provide you with feedback on your questions.
I know well the McKinsey Casablanca office so, here are the answers to your questions:
1. Yes, your other preferences will be disregarded (if the ranking you provided was 1. Geneva, 2. paris, 3. Dubai and 4. Casablanca) meaning that if you do not get an offer in the Casablanca office, you would need one year - 18 months to apply again to other offices
2. There is no such thing as global candidate pool benchmark. However, as you are interviewed, you will be "naturally" compared to your peers who are applying for the casablanca office. Be aware that if you are a good fit with them, they will give you an offer, no matter how you are comapred to other candidates. You need to stand out as an individual and not compared to others.
Let me know by PM if you have any questions about the McKinsey Casablanca office and how to prepare best for this office.
Best of luck in your interviews,