the whole objective of every case is to find the root causing issue, isolate it and provide recommendations to fix it.
So regarding your questions:
I. be more specific about your questions and work with hypotheses:
(1) i.e. is this a company internal problem or this is through changing environmental environment (i.e. market crisis, regulatory, political change, etc.) - do you have any data on that?
(2) provide the options for decreasing profitability: (a) inreasing costs, flat revenues (b) decreasing revenues flat costs (c) increasing revenues and overincreasing costs and (d) decreasing costs and overdecreasing revenues - to test these scenarios, ask for data
II. If your structure is MECE no matter which frameworks you combine and use it covers all the aspects (incl. political, societal and demographics). In your case you could also bring a substructure to answer the question what are the drivers of # of weddings? Again work with hypotheses to answer the question, why the # of weddings is dropping until you find the right answer
III. Examples of the questions might be:
-what is the proportion of males and females throught different age groups in this society / place? - Here you should already get a hint about mismatch between males and females in the specific age group
-Are there any specific reasons for the disproportion in this age group?
-You would probably asked to brainstorm about the reasons at this point to assess your creativity of answers - it's always good to brainstorm with some initial structure and not simply out of the blue
I hope it helps you.