I had been wondering about Abyssal Drain myself recently too. I agree, decouple it from Carve and Spit is the right move to make.

BUT at the risk of making it too homogenous with paladin, I wondered if it might not better be suited to being a third move in their AOE rotation? It may need adjustments to work, but giving dark knight a self heal on every third move of their AOE combo would help with self sustain and bring it more in line with the other three tanks for dungeon runs.

Also to your other point, I don't think decoupling them would be TOO difficult considering when the job launched, the two were separate skills, they were only combined into sharing a cooldown later.