A) The high temperature limit is OK, so don't change the lattice constant: we have the right total number of modes. At low T we need the curve shifted to the left: we need to lower the frequencies by a factor of two or increase the mass by x4.
B) Decrease the lattice constant by a factor of (cube root(2)) to double the number of modes to get a decent fit at high T. At low T in the Debye model, all that matters is the velocity of sound. To decrease the heat capacity by x8 requires an increase in the velocity of sound by x2.