Looking at the case, it does seems that the car density data for Germany is intentionally left out as it is designed to test market sizing capability of the prospective candidate.
This is a very common part of every case interviews. Most of the time, when we inquire to the interviewer on a specific data, they might say that the data is not available and expect us to estimate a number based on logical approximation.
The best course when facing such situation in real interview is to ask if the data is available, and if not, mentioned that since it is crucial to have the data to assess the attractiveness of the market, you will conduct a market sizing estimation.
On your second question, clarifying on unit details and any other unclear information is a common practice within a consultant's work. It is expected out of every candidates to inquire on such items during case interviews. Rest asssured that such actions will never be held against you.
Hope it helps.