So for Ilvl250+ Tank Groups i would suggest 2 Tanks and 3 Heals. (3 Heals for the reason that it's much safer). This will also make your fight very static and cleaner and you can pretty much ignore most of the abilities of the Encounter as 3 Healer just need to heal 1 Tank and hot the other.
However i am sure the tactic with 1 warlock 2 melee tanks and 2 or 3 heals is also possible, but more complex and thus just harder
We've been doing this on 10-men with 1 normal tank, 1 ranged tank and 2 healers since the encounter got unlocked and it never struck us at healing heavy. 80% of the raid damage in the encounter is perfectly avoidable and using 7 DPS to kill them makes it fairly quick as well, ensuring you have to worry less about bad kinetic bombs and overall attrition.
We tend to stick a paladin on the fire and kinetic princes and a shadow priest on Keleseth. Once the shadow priest has gathered 4+ nucleii, vampiric embrace and a few dots will keep him up forever as long as Keleseth is not the empowered target. I imagine an affliction lock can do something similar, although he will lack the 15% flat damage reduction.