Eugene, some really good questions in here! Let me try to address them.
First, can you 'overthink'? Yes of course! I am not sure this was really the issue in your particular case though: sometimes, it does come down to practice-practice-practice
Second, can you ask questions? "Yes but": you can ask some clarifications, but I suggest you think twice before asking for help. A better strategy would be to say something like "Is it fair to assume ...?", as in "I see that production of X has gone up over the last couple of years but inventory has remained constant, is it fair to assume the company was responding to a shift in demand?"
Third, and most importantly: what can you do about it? Here are three suggestions:
- Practice some more
- Read and reflect on a regular basis on all the feedback you have been given, and review the things you missed in the past. I was guilty of not doing this, and it probably cost me a few weeks' worth of prep time. Be smart about it, leverage the feedback you have been given - especially that of folks who get an internship / job at top consultancies, since we know what to look for and our advice is (usually) more actionable as a result
- When you look at exhibits, follow a strict and repeatable process. I always recommend you include 3 distinct steps: (a) Give a high level description of what the exhibit talks about, what its purpose is; describe the rows and columns; (b) dive in a little deeper, explain what the information tells you; (c) last and most important, give the insight. What is the story? Why is this important? How does this confirm (or invalidates) your hypothesis, and what is the next logical step based on that new insight?
During my 1st round at BCG, I didn't exactly walk on water: I actually performed somewhat poorly on the first case (with a consultant), but the second case (with a Principal thankfully) was a series of 6 or 8 exhibits I had to discuss. By acing that 2nd case, I got to the 2nd round and eventually got an offer. Understanding and leveraging exhibits is one of the simplest things you can do to ready yourself for the case interviews. Looks like you are already focused on them; that's good, keep working and don't despair. Good luck :)
Hope this helps. Let me know if you have follow-up questions. Thx,