EVIL Quartermaster unfortunately didn't fit so he's also good replacement. Eventually Coerce for having more spells. Silas Darkmoon also really good for opponents Rattlegore, but if you add him, then add EVIL Quartermaster as well, because you rather wanna give him lackeys.
That's why you have cards like holy nova, breath or penance- and then on turn 6 when opponent is pouring out it's time to play cabal to get advantage and win. It works, i played many games.
For combo with Wave of Apathy. The best catch is Rattlegore . Basically any problematic minion because this combo is like mind control. Against DH i played it many times to steal his Bonechewer Brawler and that gave me big advantage. You can replace it with Cabal Acolyte but i think Cabal Shadow Priest is just better.
I think if you wanna play c'thun you, better focus all strategy on surviving and copying him with elise, so don't cut yogg because he's not pure win condition and works nice with c'thun. Remove clown package wich is yshaarj clown and survival. and add something like omu and this 4 mana with taunt that heals.