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try timing your fade when the adds spawn. i believe they do a fixate (based on threat) when they spawn, so healers get picked a lot and fade won't help after they fixate.
i don't think renew is a good idea as disc during black phase. the amount of healing you get per stack is not likely to be worth it, and only holy can refresh it for free in serenity. you will get another stack if you refresh by casting renew again.
you should be able to blanket the raid in PW:S before the purple hits the boss and pump up some good DA's with PoH as well. PW:S only gives 1 stack, even when glyphed, so it seems like a good use of a stack. pennance gives 1 stack, not 3, so it's viable. you basically get four heals on each player every ~30 seconds (length of the debuff) to keep your group alive.
you could also try 3 healing; it's not necessary, but it does make it a bit easier if you have enough dps.