Using two sheets, one for clean and one for "dirty" work is generally a good idea to separate your structure and thinking from rough calculations. To answer your questions:
1) You will not be evaluated on your ability to take clear notes when you are given the initial prompt. What is important is that you can understand it - therefore, use the least possible level of detail so that you can still clearly understand the key points". I would always take these notes at the top of the "clean" paper, as they form the context of the entire problem and should be referred to throughout the case.
2) Similar to above: as long as it is clear enough that you can verbally explain it, it's fine. I certainly wouldn't fully write it out (e.g. "Profitability has decreased due to increased costs")
3)This really depends but I would generally write out the full issue tree which you intend to take your interviewer through in your opening structure. It is likely that during discussion this will be expanded, at which point it is perfectly fine to expand it further.
4) I would recommend to first think of categories, and then write down specific ideas. For example, if the question is "how are some ways this MNC could increase its profits" you might first break it down into:
- Existing business
- New business
- New markets
- Ideas: acquire, organic, JV, etc.
- Existing market
- Ideas: acquire, adjacancies, etc.
5) This depends on the case. When the interviewer asks you to make a final recommendation, ask if you can have a minute to structure your thoughts. Some interviewers will want you to answer on the spot (pressure test), in which case you shouldn't write anything. If they give you a minute, then you should definitely write out the (brief) structure exactly as you say with supporting points. To shine in the interview it can also be recommended to add some additional colour on top of the standard "Conclusion is X because of A, B, C" - for example, you might lay out "low hanging fruit" vs "long term goals", or suggest immediate next steps / what further analysis you would want to do if you had the time.
6) It is not worth wasting any time transcribing things from the "dirty" to the "clean" paper. Dirty should be used primarily for rough calculations, while clean is for your structure, notes, and cleaned numbers.
Hope this helps!