The Emissary is a good buy, for all the reasons you mention, i have trouble building lists without it.
I'd suggest swapping evidence tampering with plastic surgery and adding Moonlighting, turning everybody nearby undead is great for rancid transplant and guaranteeing Flesh constructs off anything with expunge, and the push when Mcmourning takes poison damage is really handy.
I'd drop transfusion off Sebastion, rancid transplant and plastic surgery should be enough to get you stacks of poison to expunge, and is a lot harder to counter.
I've found Rafkin too squishy for decaying aura, my little helper could be good on either Rafkin or Sebastion to help with survivability.