"Longevity" is a general and imprecise term. It means "living for a long time." "Aunt Edna was blessed with longevity and lived to be 104 years old; Uncle Bill was not and died in his forties." "Life expectancy" is a precise term with a mathematical meaning. It is a term used by "actuaries," people that calculate premiums for life insurance companies. It means the average number of years that a large, specific population of people lives. For the entire population of Japan, the life expectancy at birth is 83.7 years; for the entire world, 71.0 years

Yes. Life expectancy is the average period that a person may expect to live (in some places people may not expect to live very long). Longevity means “long life”: “the greater longevity of women compared with men”.