Why can we view diamond structure as FCC with a basis of 2 or as S.C with a basis of 8?

Answer 1

There are 8 unique positions in the unit cell, so the basis is 8 for SC. FCC generates 4 symmetry related vectors
x,y,z
x+1/2, y+1/2, z
x+1/2,y,z+1/2
x,y+1/2,z+1/2
which together with the basis (0,0,0) and (1/8,1/8,1/8) generates all of the positions.