This is not easy! The answers are "is", "are" and "lives", although many people will use either "is" or "are" in 2 and 3. as well, I believe. I think the rule would be, if there is one, is that "one fifth" is singular even though there are many people contained within that one fifth. But in 2. and 3. the emphasis can also be more on the individual members of the population and so to emphasize that, the plural form works. 1. has to be singular because you are saying, in effect, one-fifth "is equal to" or just "is" for short, two million. I fear that may not be very helpful, but maybe someone who knows grammar can respond.