There are a zillion different conflicting strategies on here and I think many of them are not supportive of your comp ("get a hunter" is not helpful), but I think with your comp, you're doing the right strategy.
Things to note:
- Single tank worm.
- DK off-tank goes frost spec with chillblains and kites the adds. DPS ignores them. If you're kiting the adds all fight with a tank, there is no need pull any other DPSers away from the boss to kill them. Kite, kite, kite. They should never hit the tank, so it shouldn't matter if you have a zillion of them.
- Stack everyone else in melee and have them all pound on the boss. If you're kiting and not killing adds, you don't want your ranged scattered around the room.
- Your healer mana situation sounds about right. It is scary to be that low, but they will be regenning in phase 2 when there is no healing needed.
- Your DPS is way too low. Work with your DPSers to get them higher, whether its to go over the latest theorycrafting for them or troubleshoot particular problems. See how they do, though, if they're focusing on the boss full time. If they're not getting higher, then something needs fixed in this area.