1: Lightning: Apocalypse is not a good set to 6-slot, because you're wasting a ton of Damage slotting from it. I don't think that the Psy resist is worth it, especially for a /Dark build. Either 5-slot it, or use the 6th slot for a damage IO, or for a pure Endurance IO if you're having Endurance problems.
2: No Dark Servant? Unless you're absolutely set on not having it for character concept reason, the power is basically a must-have. Put it this way: with Dark Servant, you can solo AVs with this character.
3: Both Power Sink and Conserve Power? I think that's overkill, especially with Electric Blast, since it's the most End-friendly blast set. On the other hand, the Mu Adept is a nice power to have. I'd definitely drop one of the powers for it. Normally I'd keep Conserve Power, but the combination of Short Circuit/Power Sink to drain spawns is interesting to consider.
4: Ball Lightning: same comment as Lightning. Drop one of the Ragnarok IOs, and put the Positron's damage proc IO.
5: Fearsome Stare: do not slot this for Fear. It's long enough duration. Slot it for to-hit instead. 4x Cloud Senses gives you a nice 6.25% Recharge bonus.
6: I'm not sold on all the slots for Combat Jumping. It's Defense is not all that high, and its End cost is pretty low. I always either leave it with a single LotG, or add the Kismet Accuracy IO, no more.
BTW, with Dark Servant, the character should most certainly be able to solo AVs. Using Twilight Grasp and Darkest Night, and Dark Servant's damage debuffs, you should lower the AV's damage down to 20-30% of its normal damage. Add in the resists and being at the soft cap for all types of attacks (after all of the to-hit debuffs), and the character should have no problems surviving in front of basically any AV that you meet in solo arcs. You might want to see if you can put in Hasten and increase the Recharge in Howling Twilight for that, since it's a major -Regen debuff, but defense should not be an issue.