  The electronic structure and half-metallic gap of Co$_{2}$MnSi in the
presence of crystallographic defects, partial Fe substitution for Mn, and
thermal spin fluctuations are studied using the coherent potential
approximation and the disordered local moment method. In the presence of 5\% Co
or Mn vacancies the Fermi level shifts down to the minority-spin valence-band
maximum. In contrast to NiMnSb, both types of Mn antisite defects in
Co$_{2}$MnSi are strongly exchange-coupled to the host magnetization, and
thermal spin fluctuations do not strongly affect the half-metallic gap. Partial
substitution of Mn by Fe results in considerable changes in the Bloch spectral
function near the Fermi level, which strongly deviate from the rigid-band
picture. In particular, a light band with the Fe character crosses the Fermi
level at about 50\% concentration. At room temperature, Fe substitution of up
to 30\% slightly increases the spin polarization at the Fermi level.
