How to write a strong research plan?
With your research plan, you need to convince a recruitment committee that you have a viable plan from which to launch your career.  

Your plan will need to show the committee  


that it is scientifically sound
that you have the skill set to carry it out (or know how you will acquire it)
that you understand what resources you need to carry it out
that you understand your funding streams
that there is something that gives you competitive advantage among the dozens of other people in the field vying for money from the same pool as you.


The weight of each of these considerations will be a function of your complete package.

So, the generic answer to all of your questions is "do what you need to do to accomplish the list above"

I suggest you work on your plan and carefully review it with some mentors.

As to the level of specificity, you shouldn't have to lay out complete experimental protocols, but you should be prepared to discuss each proposed line of experiments at that level.  In your statement, though, it would be quite useful to have specific hypotheses for each research path.

You should also be prepared to prioritize.  When asked about how you would spend the next five years, you should know the answer, and pretty specifically.