#1 Rule: Ask questions that help YOU understand the world.
If you're confused, figure out why and ask the question that clarifies (normally this is around how the business actually operates, or the real challenge they're facing).
Forget about the classic questions. Think for yourself and actually drive your case. Imagine yourself at the client site, with your team, in front of a whiteboard, figuring out the answer. Too often, candidates have a pocket list of 10 questions that they're seen/heard over and over again, and they repear them without actually understanding why they're asking them in the first palce.
In regards to your questions:
1. This is so generic. Add color to this. I.e. for External vs. Internal "I'm thinking this could be due to something going on inside the company (maybe operational issues, change of management, something like that) or something outside of our control such as price shocks, new competitors coming in, or a generally declining demand. Do we have any information on this?"
2. Your job is to figure out why. Your framework doesn't end after the initial prompt + clarifying questions. You need to continue to use frameworks and strucutred thoughts throughout the case. DRIVE to the answer by using a structured approach. What could be causing this and what information do you need to figure out the why? In short, yes, come up with another structure (external vs internal could work)
3. You just need to have a structure that covers this. If you've taken a MECE approach, this would be covered. For example, if you did external vs. internal, in your external segment it's not unreasonable that you would put: Competing products taking their interest away, emigration, conflict/political issues, etc.