Thanks for sharing your experience - it's important for others to see this and for you to learn from it!
I think this point of yours is the most interesting: "I found my case to be extremely odd and personally unpleasant for me"
If you want to succeed in not just recruiting, but also real-world consulting, you need to suck it up.
Sorry to be so blunt, but once you join MBB you WILL have some projects that are horrible. You WILL have some unbearable clients. You WILL have a manager/partner that is incredibly demanding, seemingly changes their mind, doesn't articulate clearly, etc.
(^not all projects/clients/managers, but enough)
In both consulting and interview preparation you need to be ready and prepared for any scenario.
This is why I teach "learning how to learn". Memorizing frameworks, planning for a specific, standardized process, etc. is not the way to approach interviews! I am very sorry that you learned this during your Mckinsey interview, but luckily you can move forward from here with your learnings.
Two final points:
1) McKinsey interviews are notorious for being particularly tough/demanding/"mean"
2) You definitely missed a key part of the structure, so unfortunately they were indeed waiting for a specific answer
The greatest accomplishments come from temporary defeat. Learn from this and move forward!