To best achieve the mana return when spamming DDs, ensure there is enough range between the warlock and the target so that Drain Soul can be cast whilst the DD is reaching it. This is easy on caster and ranged mobs. For melee mobs, let the Voidwalker or Felguard keep it at the distance. Reduce the mobs health such that it is one shot from death. Send in the executing shot and follow immediately with a Drain Soul. The DD flies slowly, and the Drain Soul connects instantly, before the DD lands and kills the mob.
If you have problems with too many Soul Shards, addons like Necrosis have support for automatic deletion.