Both approaches are not very good. What is extremely important to understand in order to craft super strong structures: the purpose of a structure is not to provide a list of things you want to look at.
The purpose of a structrue is to clearly explain how you will answer the precise question that has been asked by the client.
None of the two approaches which you outlined above caters for this! The client's question is
"How to initially determine the cause of the revenue issue?"
Telling the client what you want to look at does not address this question. Frankly, you may look at whatever you want, but this does not help the client understand, HOW you will answer his concrete question. In order to address this question, you have to outline the LOGIC according to which it can be answered. And this is fundamentally different from just defining buckets and topics. (And to anticipate your next question: yes, the solutions provided in popular case books and even on the MBB websites are oftentimes very very weak.)
Instead of falling for such weak thinking, you should outline a clear logic that will ultimately and invariably lead to the root cause of the revenue problem.
- For this, you first need to isolate what (sub-)driver causes the decline. Revenue is too high level, you need to find out whether it is an issue with pricing, or the product mix, or the quantities. If, e.g., lower quantity is the problem, then you drill deeper to understand the concrete numerical driver (e.g., the average number of items per purchase has not changed, but the number of purchases has gone down --> then you drill deeper to understand what is driving this (the "sub-driver") --> e.g., the number of customers has not changed, but their average frequency of purchasing has gone down --> this is the numerical problem driver! You isolated it just by means of a driver tree).
- Once you have isolated the problem driver (WHAT is the problem?), then you check on the qualitative reasons that might have caused this very problem driver to develop negatively (WHY does the problem exist?). You exclude all other areas of the tree because they are not relevant! This is how you run effective and efficient diagnostics. This second step of qualitative analysis might indeed require some extra structuring once you reach it!
What aspect that is very important (and usually violated in Case Coaching books) is the principle of first isolating the numerical problem driver, before asking qualitative question. Never start your analysis with asking qualitative questions ("First I would like to get a general understanding of the market development" and such phrases)! This is practically the very definition of "boiling the ocean", i.e., working in an extremely inefficient way. First, you should seek to narrow down the area that you need to qualitatively understand - and this can be done very quickly by doing a numerical analysis as described above. Once you know where the problem comes from, THEN you can start to understand the qualitative reasons that underlie the negative development of this driver, and this analysis will be far more focused and concrete than if you would have tried to do it at the start.
P.S.: Whetehr the case is conducted in an interviewee-led or interviewer-led manner is absolutely irrelevant for the initial structuring of the case! It just comes into play once you enter the analysis.