Olivia, in my opinion, is interchangeable. She can be pulled in favor of more spells, such as Putrefy or Searing Spear.

Rakdos's Return can be swapped out in favor of Jarad's Orders in order to get the right creatures into my hand and into the graveyard to fuel good ol' Varolz.

One of my 4 drops--either Desecration Demon or Hellrider--can be swapped out for Ground Seal if I'm having troubles with graveyard hate.

Abrupt Decay is a good all-around removal, but may have issues for late game play. I'm on the fence about that card, in all honesty, but I think, for now, it works.