Why does the Torah mention here the fact that Hashem blessed Adam and Chavah?
Ramban: Adam and Chavah were created, not born to parents. Consequently, their ability to have many children (which are currently under discussion) was a natural phenomenon, but the result a Berachah that followed their creation.
Seeing as 'Adam' is a general term that pertains to all of mankind, why did Hashem call Adam and Chavah by that name?
Ramban: Because they were the blueprint from whom all of mankind descended.
Why does the Torah discuss creation of man in the singular and in the plural?
Peninim mi'Shulchan ha'Gra: Hashem's intent was to create two, so woman would be a helper for man. In actuality He initially created them as one, so there would be love between them.