Malign Spell Focus gives +2 DC and Spell Focus (Evil) gives +1 DC, I think they are different, but if they are the same then you loose +2DC, because Spell Focus (Evil) would be the version you use.
Corrupt Spell is in Complete Divine, the wizard substitution levels are in the Planar Handbook, Fiend of Possession and Fiend of Corruption are in the Fiend Folio, and everything else is in BoVD.
-Also, if you get a Soul Tick from Fiend Folio it will automatically empower all of your evil spells, which should be everything. Nice.