Some input on my side on the two points you mentioned
Q1: The case question is very open ended, so I think all the categories suggested in the solution are potential ways to segment the market. However, should I have to estimate the potential market size for our company I would also consider car owners vs. non car-owners, then, as you suggested, looking into each of the two categories and for example estimate:
- for car owners how many of them are in the income range that can afford our electric car, then how many of these would be willing to switch to electric, how many of these are interested in our type of car (i.e. is our car an electric SUV or Luxory or something else). Then based on the average life span of a car in the US you can estimate how many of current car owners that are potentially interested in our car would buy a new car in a year.
- for the non-owner you would make a similar reasoning probably taking into considerations that some of theme would never buy a car (too old/too young, not interested, not having a license, etc)
Conclusion: here some observations
1) In general, you do need to pick one path to solve a case. However, at the beginning of the case you should clearly specify why you decid to pick a specific path. For example (may not really apply to the specific case, but just to give an idea od the type of sentences you should use :-) ): " I think that in order to estimate the market for e-vehicles in the upcoming X years I should consider both current car owners and non-car owners, but for the sake of this case I think it is safe enough to only consider car owners because..."
2) When it comes to drawing final strategic conclusions at the end of the case you can say that based on the analysis you have just performed you would advise the company to undertake a certain action. However, you should also mention that there is a number of other factors that weren't analyzed during the resolution of the case, but that should be taken into consideration in a real project work and that could possibly impact the strategic advise you give to the client.
The interviewer wants to see if you are able to think beyond the sometimes straightforward solution of the case and take into considerations the complexities that arise during real projects. But do not be stuck in complexities! It is good to mention them, but still you need to pick a path to solve the case.
Hope this answers your question!