Some answers to your questions (the best I can remember because I can't seem them while replying):

In Phase 1 here is what my raid does:

Melee DPS - Stays on Anub.
Ranged DPS - Burn down the Burrowers as soon as they spawn then back on the boss

So when phase 2 starts, we don't have any Burrowers up. Normally our ranged are good enough at taking them down fast and having them melee on them makes the fight last too long so we just let them burn into him.

Phase 2:

He goes underground and the scarabs start spawning. Assuming you killed all the Burrowers, there shouldn't be any in this phase. Basically we just have all dps burn down the scarabs. We tell people to kite them and not get hit if possible. So the ranged try to get aggro on all the scarabs and the melee follow them around and kill them.

Whoever is tanking Anub should definitely avoid getting any of the debuffs because we don't need him taking bonus damage.

So to answer that question, all dps are killing adds, healers are healing the raid, and tanks are helping dps (not tanking). If you get targetted, you kite the spikes through the ice.

Something that is not mentioned in this article is that phase 1 and 2 rotate back and forth. He burrows for a certain amount of time, and then he pops back up on his own until you get him to 30%.

Yes, the spikes will kill the Scarabs, but we usually just let that happen naturally as they die quickly anyway and we don't want our people getting spiked.

Also, in phase 3, my raid keeps the MT at full health, because it is too easy for him to go down. Everyone else is below 50%.